From the Super Mario Wiki
“(BOO! Did I distract you?)”
Boos (originally known as “Boo” Diddlys and later Boo Buddies) are common ghost enemies in the Mario series that first appear in Super Mario Bros. 3. They are a type of white spherical ghost with menacing and constantly open mouths. They are known for their large tongues and small "arms" of some sort. However, the most distinctive characteristic of the Boos is their shy personality, where they cover their faces when someone looks them directly at the face. They will uncover their faces once the subject turns around. Goombario's tattle for the Boo Igor in Paper Mario states: "He probably was a merchant before he became a Boo," implying that current Boos may have once been living beings. It is revealed in Super Mario Galaxy that they can speak in Booish.
Super Mario series
Super Mario Bros. 3
Boos first appear in Super Mario Bros. 3. Here, they are referred to as “Boo” Diddlys, a pun off the late rock star Bo Diddley. However, future games refer to them only as Boos. The Boo shields its face if Mario or Luigi look at it, but it starts to chase Mario or Luigi if they look away. Boos are unable to be defeated unless the two are equipped with a Hammer Suit, Koopa shell, or Starman. The game also has Stretches, which are Boos with very long bodies that live in the Fortresses throughout the Mushroom Kingdom.
Super Mario World
In Super Mario World, there are Ghost Houses, which contain many Boos (referred to as “Boo” Buddies). These Boos retain their usual pattern of attack from Super Mario Bros. 3 and sometimes make faces at Mario or Luigi. However, they are somewhat slower and less accurate than in Super Mario Bros. 3. This game also introduces many new sub-species of Boo, such as the Boo Buddy Block, Boo Buddy Snake, Boo Buddy Swarm, Circling Boo Buddies, Eeries, and Disappearing Boo Buddies. It also introduces a giant version of the Boo, the Big Boo, and a ghost-like Lakitu called a Fishin' Boo. One Ghost House contains a secret boss, The Big Boo. The only way to defeat a Boo is with Super Stars or with Grab Blocks when he is visible.
Super Mario 64/Super Mario 64 DS
Boos make a starring appearance in the course Big Boo's Haunt in Super Mario 64. A group of Boos can be found in the Castle Gardens (eight of which are holding Red Coins in the DS version). After collecting twelve Power Stars, Mario is able to enter Big Boo's Haunt by going into the little cage that a certain Boo drops; this Boo will be somewhat bigger than the others. He also battles Big Boo three times here. Additionally, when Mario looks at the Boos, they attempt to hide by becoming partially transparent, but if Mario is not looking at them, they chase him. If they hit him, he loses two wedges of health. In the translucent Boo, the player can see a Coin or a Blue Coin. To defeat them, a punch or kick from behind or the side is needed. They can also be defeated with a Ground Pound while he is not looking at them. When defeated they give a Blue Coin, but the ones in the Castle Gardens give a normal Coin instead. When defeated in the first mission, "Go On A Ghost Hunt", the Boos will say, "Ghosts... ...don't... ...DIE! Heh, heh, heh! Can you get out of here... ...alive?" when defeated. In addition, a Boo will appear in the hallway of Peach's Castle after twelve Stars are collected.
In Super Mario 64 DS, in Big Boo's Haunt, Mario can find a Luigi painting, which will take him to the Big Boo Battle level on the balcony. After reaching the end by following the cackle of King Boo, he makes it to a room with a mirror at the end of it. He then has to battle King Boo. However, King Boo is invisible, and the only way for Mario to see it is by looking in the mirror, making it harder to hit the Boo. After hitting it three times, Mario gains the key to the room in which Luigi is imprisoned and proceeds to free him. In the English version of the game, King Boo is erroneously referred to as "Big Boo." Yoshi cannot defeat Boos in this game. Ground Pounding yields no results, and eating them simply causes Yoshi to spit them back out. Fortunately, Mario, Luigi, or Wario can hit Boos with ground pounds.
Super Mario Bros. Deluxe
While Boos do not appear in the original Super Mario Bros., Boos appear in Super Mario Bros. Deluxe as the main enemy in the "You VS. Boo" mode. The goal is to reach the flagpole before the Boo. After defeating Boo a certain amount of times, it flies away, and a Green Boo replaces it. After that Boo is defeated, a Red Boo takes its place, and finally, a Black Boo with a yellow outline appears. Each new Boo is faster than the one before it, meaning the plain White Boo is the slowest, while the so-called "Legendary Black Boo" is the fastest and hardest to defeat.
Super Mario Sunshine
Boos return in Super Mario Sunshine, terrorizing the residents of Hotel Delfino. In this game, they can be stunned by F.L.U.D.D. and defeated by being jumped on, making Sunshine the only game outside of the Mario RPGs and Super Mario 64 where jumping can defeat them. Boos in this game can also be eaten by Yoshi. They also disguise themselves as Yellow Coins & Blue Coins. Eventually, he finds King Boo in the basement of the Casino, and Mario has to battle him. He can defeat him by throwing a Chili Pepper at him and then throwing another piece of fruit at him. Mario has to do this three times before King Boo is defeated. Boos can also disguise themselves as Shadow Mario, while the real Shadow Mario runs from Mario. Boo's appearance has just one change, which is blue circles surrounding their black pupils. Pink Boos also appear and can be turned into a platform if sprayed by F.L.U.D.D. Also appearing inside the hotel are a new species, known as Sleepy Boos. These Boos mostly appear to block passages. F.L.U.D.D.'s water does not affect them; they can be removed only by being eaten by Yoshi.
New Super Mario Bros.
Boos also appear in New Super Mario Bros.. They are found mainly in Ghost Houses, scattered throughout the game as well as World 8-1. They appear like they usually do, shy, and they act the same as in Super Mario Bros. 3. They can only be defeated with a Shell, a Starman, a Mega Mushroom, or a Blue Shell. The game also introduces the Balloon Boo, which are normal Boos who can suck up air, and become much bigger. Broozer, the boxing ghost, first appears in this game as well. An icon of 2 Boos covers the progress map in Ghost Houses.
Super Mario Galaxy
Boos make an appearance in Super Mario Galaxy and can be found in the Ghostly Galaxy, the Bigmouth Galaxy, the Deep Dark Galaxy, and the Sand Spiral Galaxy. Boos act the same way that they did in previous games, such as hiding by turning transparent when Mario looks at them and then following Mario when he looks away. Boos can be defeated only when they are brought under a ray of light (including the Toad Brigade captain's helmet and the light emitted from shells while in the water) or by Rainbow Mario touching them. The spin attack, however, can throw a Boo around Mario, knocking it off course temporarily or into a light. Boos can also be found underwater in the Bigmouth Galaxy where they attack just by swimming back and forth, similar to Cheep-Cheeps. In Deep Dark Galaxy, in the secret mission, there is a Boo that has a Power Star in it.
If a Boo spots Mario while he is a Boo, it develops an attraction for him and follows him. If a Boo hits Mario, Mario loses the powerup, but does not lose any health.
An individual Boo named Spooky Speedster is found in two areas of the game. Mario must race him twice to win a Power Star. He is raced once in the Ghostly Galaxy and once again in the Boo's Boneyard Galaxy. This game also introduces a new kind of Boo, called the Bomb Boo; it is similar to a normal Boo, but is black with yellow eyes and can be grabbed by its tongue if Mario uses his spin attack on it.
New Super Mario Bros. Wii
Boos reappear in New Super Mario Bros. Wii in ghost houses throughout the game. They behave the same way as they did in New Super Mario Bros and they are invincible, unless the player has a Star power-up. Big Boos also reappear along with Broozers. Circling Boo Buddies can also be found in a small amount of ghost houses in the game. Despite their concept art, they appear with a blue glow as opposed to plain white in this game, giving off a more spectral appearance. In the map, a single Boo will be watching Mario, and will taunt Mario as he enters. If Mario gets tossed out, the Boo will laugh. However, once the level is beaten, the Boo will cover its face for the rest of the game.
Super Mario Galaxy 2
Boos reappear in the game Super Mario Galaxy 2. They behave the same way they did in its predecessor. They are found in haunted levels such as Haunty Halls Galaxy and Boo Moon Galaxy. The underwater Boos from the predecessor also appear in this game, in the Battle Belt Galaxy and Slimy Spring Galaxy. Bomb Boos and Big Boos also reappear.
Super Mario 3D Land
Boos later appear in Super Mario 3D Land. In the game, they retain their normal battle strategy, which is chasing Mario when he looks away while sticking its tongue out. Boos in this game also momentarily disappear when hit with Fire Mario's fireballs and Tanooki Mario's tail whips. Boos also laugh in their original phonic from Super Mario 64. Some Boos have Tanooki tails; these are called Tail Boos. They act like normal Boos, but, when they stop moving when Mario looks at them, they will swing their tails once, which can damage Mario. Big Boos also appear during the game. Another species of Boo is also introduced, called Peepas. They do not act like normal Boos, instead moving around a set position, and do not halt when Mario looks at them. They act the same as they do in Super Mario Galaxy.
New Super Mario Bros. 2
Boos, along with Big Boos, return in New Super Mario Bros. 2, with the same appearance and behaviour as in New Super Mario Bros. Wii, except they turn completely red when Mario faces them. They can be found in Ghost Houses along with other ghost enemies such as Peepas and the new Boohemoth. All of these varieties, save for Boohemoth, can be defeated with a Star or Gold Flower. Boos can also be turned Gold Boos with a Gold Ring. When gold, Boos float away from Mario, leaving a trail of coins behind them.
There are also Boos that push walls in a Ghost House. Here they push the wall either left or right and they do not stop when Mario looks at them.
New Super Mario Bros. U
Boos, Big Boos, and Circling Boo Buddies appear in the Wii U title New Super Mario Bros. U. Their behavior is the same as that of past installments. Boos also have their own Enemy Course in the haunted area of the Soda Jungle, where a group of Boos appear in an area with various Donut Lifts, and defeating them must occupy the Super Star located inside a Flying Question Block. They can also be defeated with a Baby Yoshi.
Super Mario 3D World
Boos appear in Super Mario 3D World, along with Peepas and Big Boos. They retain the same strategy from Super Mario 3D Land and utter the same laugh from Super Mario 64. They appear to be a bit more transparent than in previous mainstream Mario games and give out a blueish glow like in the New Super Mario Bros. games, as well as a reddish glow and even steam when Mario faces them.
The Adventures of Super Mario Bros. 3
Boos make infrequent appearances in The Adventures of Super Mario Bros. 3 television series, which depicts them as having a somewhat menacing appearance. Like in the game the show is based on, Boos work for Bowser and often aid him in causing trouble. An example of this is seen in the episode "Reign Storm", in which Bowser has several Boos terrorize the new Mushroom Shopping Mall. In this episode they are referred to as "Boo Diddlys".
Super Mario World television series
In Super Mario World, Boos, who are completely redesigned, appear in the episodes "Ghosts 'R' Us", in which a gang of Boos, as well as a Big Boo, appear to chase Yoshi through Wizardheimer's Haunted House; "The Night Before Cave Christmas", scaring Yoshi away; and "The Yoshi Shuffle" in where several Boos appear in the audience during the episode. They are referred to as ghosts.
Nintendo Adventure Books
In Koopa Capers, a Boo confronts Luigi if he decides to simply look through a keyhole in a fortress door, instead of barging through it. If Luigi decides to run away from it immediately, he will successfully escape, but if he decides to stand his ground, the Boo will chase him onto some floating blocks after he dodges its attack. Making a beeline for the door after mounting the blocks will allow the Boo to catch Luigi, but doing evasive techniques will allow him to escape it. When he begins his descent into the Magma Pit, Luigi will hear a Boo behind him, which causes him to pick up the pace, and escape it.
Super Mario World: Mario to Yoshi no Bōken Land
In the interactive OVA Super Mario World: Mario to Yoshi no Bōken Land, Boos appear in a Ghost House that Mario, Luigi, and Yoshi are passing through. A Boo starts following Luigi, who is in the back, but every time he turns around, it disappears. Finally, he quickly does a double take, and turns just it time to see it re-materialize. He panics and Boos start swarming the group. Mario grabs a candle and lights it, which repels the Boos, and one drops a key they need to progress.
Boos make an appearance in Yoshi. They are enemies in the A Type part of the game, as well as the B Type. Boos are not main enemies in the game, as they simply make appearances throughout some parts of the game. Boo's sprite in the game is similar to its sprite from Super Mario Bros. 3. Many of the other enemies' sprites also are similar to their sprites from Super Mario Bros. 3. In the North American commercial for this game, they are referred to as "Boo Buddies", when it is referring to the normal standard Boo instead of the different versions.
In Yoshi's Cookie, Boos return as enemies to Yoshi. Though in this game, they are depicted as one of Yoshi's "opponents". They are among the easy opponents, the others being Goomba, Cheep Cheep, Paratroopa, and Spiny. Two sub-species of Boo, Eerie and Fishin' Boo appear, too.
In Yoshi's Safari, Boos appear as enemies. They first and only appearance throughout the game is in the Ghost Mansion, a level in the Dark Realm. A Big Boo serves as the boss for the level. Just like any other enemy in the game, Yoshi must shoot the Boos with his Super Scope.
Super Mario World 2: Yoshi's Island
Boos first appear chronologically as antagonists to Yoshi in Super Mario World 2: Yoshi's Island and its remake Yoshi's Island: Super Mario Advance 3. They are first seen in World 2, which features numerous ghastly creatures. They are underlings for Kamek, the head of the Magikoopas, who turns one Big Boo into Bigger Boo, which grows in size every time he is hit by an egg. It serves as a mini boss. The main boss of World 2 is also a Boo-like creature, Roger the Potted Ghost, who is a normal flower pot, until Kamek makes him giant, and transforms it into a living creature. The game also features the Boo Blah, a spectacle Boo exclusive to this game. The game also features the Boo Balloon, who are balloons made to look like Boos; they yield items when they pop. The only way to defeat Boos is by looking away and then make an egg bounce off a wall after it is thrown, hitting the ghost in the face.
Boos appear in Yoshi's Story in Stage 6-3: Ghost Castle. They do not act differently, although they are slightly bigger. In this game, the normal Boo is called a Big Boo, despite not being an actual Big Boo. The Ghost Castle is also home to other Boo species: namely, the Boo Block, Blindfold Boo, and others. The stage, Yoshi Sewers, also has ghost platforms, along with a blue boo variant called a Boo Buddy.
Boos appear again in Yoshi Topsy-Turvy. In this game, they only appear in four missions. They follow the same tactics as normal. In this game, they are in 3-D, and use their normal laugh that is used in previous games.
Yoshi's Island DS
Yoshi comes into contact with Boos, again, in Yoshi's Island DS. Here, they are commonly found in castles, along with other ghastly creatures. Again, they work for Kamek. They attack the same as all Boos before, but like in Super Mario World 2: Yoshi's Island, they can only be defeated if Yoshi looks away. So Yoshi must bounce an egg off of the wall into their face to defeat them. A Boo-like mid-boss, named Hector the Reflector appears as well. The Boos' laughs are used in many games before. It is used for the boss Six-Face Sal, and even Baby Bowser.
Yoshi's New Island
Mario Kart series
Super Mario Kart
Boos made their first appearance in the Mario Kart series, in Super Mario Kart. Though not as obstacles, they only appear in three race courses. These courses are Ghost Valley 1, Ghost Valley 2, and Ghost Valley 3. In each race, they just laugh when the player drives by. They also appear as items, under the name, "Ghosts". When the player uses it, they will become transparent temporarily, while the "Ghost" steals the opposing player's item, if available. This item is only obtainable in "2 Player" mode.
Mario Kart 64
Boos are used as an item, in Mario Kart 64. When activated, the Boo selects a racer with the best item in reach, and steals the item and gives it to the player that released it. At the same time, the player obtains total invisibility and invincibility for a short period of time. While invisible, the player can pass the items, the course obstacles and the other racers more easily by simply going through them. Also, the drivers who are under the effects of the Boo can run off-road without any speed loss. If no other players have any items, the Boo will return empty handed. Boos also appear in the course Banshee Boardwalk.
Mario Kart: Super Circuit
Boos appear as items, again, in Mario Kart: Super Circuit. When activated, they can also slow down the leading racer. This is the first in the series where Boos if used CPU racers can also steal items from the player as well and make the player lose Coins also. Boos also appear in two new courses, Boo Lake and Broken Pier. In Boo Lake, Boos hold candles which guide the player through the track. In Broken Pier, they attack the players and take their coins.
Mario Kart Arcade GP
Boos appear as items in the arcade game, Mario Kart Arcade GP. A Boo appears as a basic item. It causes whoever is hit by it to slow down temporarily. There is also a new item called the Triple Boo.
Mario Kart DS
In Mario Kart DS, Boos reappear as items. The only difference is, they do not slow down the leading racer. Instead, they steal the nearest racer's item. Also, using the Boo won't affect off-road racing, so various shortcuts can be made. Some pictures of Boos from the game, Luigi's Mansion, appear in the Luigi's Mansion race course. Banshee Boardwalk reappears as a race, along with the Boos. They do not do anything that affects the racer; they just laugh when a racer drives by.
Mario Kart Arcade GP 2
Boos reappear in the arcade game, Mario Kart Arcade GP 2. Like what they do in Mario Kart Arcade GP, they slow down the leading racer. Though, the Triple Boos do not appear in this game, even though it appears in the predecessor. However they do not appear in Mario Kart Arcade GP DX.
Mario Kart Wii
Boos do not appear as items in Mario Kart Wii, but King Boo appears as a playable character. However, Boos do appear in a returning course, Ghost Valley 2. Like their original appearance in the track, Boos only laugh when racers drive by, but they eventually disappear. Boo also makes a cameo in Bowser's Castle. At the end of the track Boo's Mario Party 7 artwork can be seen along with Dry Bones' artwork.
It appears that Boos have been permanently removed from Mario Kart as items as of this game as they have not reappeared since.
Mario Kart 7
In Mario Kart 7, Boos appear in the returning track, Luigi's Mansion, in place of the portrait ghosts. They will pop out of their paintings when the drivers race by them.
Mario Kart 8
Super Mario Land 2: 6 Golden Coins
Boos appear in Super Mario Land 2: 6 Golden Coins. These Boos are relatively unchanged from the previous Boos from Super Mario Bros. 3. Boos in this game are found in the Pumpkin Zone. A thrown Green Shell can defeat it. A new sub-species named Terekuribos, which are a species of undead Goombas, also appears in this game.
Super Mario Adventures
In Super Mario Adventures, a horde of Boos appear to attack Mario and Luigi when they are tricked into a haunted house by Bowser and the Koopalings. Mario and Luigi become trapped by the Boos and a Big Boo, who are held at bay by the bros staring at them in the face. Eventually, nearing exhaustion, Mario races to a nearby room, dragging Luigi while being chased by the Boos. Upon barging into this room, the Boos and Big Boo find Mario dressed as a psychiatrist with Luigi as his nurse. After some talking, Mario and Luigi manage to reform the Boos and the Big Boo, while learning they were bullied in their youth (which is why they act so nasty to everyone). These Boos and their Big Boo leader were last seen happily waving goodbye to Mario, Luigi and Yoshi as they left the haunted house, which the Boos led them out of.
Boos also make an appearance at Peach's wedding.
Boos make an appearance in the Super Mario All-Stars remake of Mario Bros.. This version of Super Mario All-Stars is for the NES. Koopa Troopas, who replace Shellcreepers, and Spinies also appear in the remakes of Mario Bros., but not in the same remake as Boos. Boos also replace the green fireballs from the original Mario Bros..
Boos also appear in the game Hotel Mario, though exclusively under the name of Boo Buddies. There are two types of Boos in this game. One type stays visible and does not hurt Mario until transparency is achieved, at which they can harm the heroic plumber. The other type of Boo uses the traditional attack method and turns invisible when looked at, and attacks Mario when his back is turned.
Boos also make a cameo appearance in the Game Boy game, Mario's Picross. They appear in one of the pictures in the "Early Picross" stage in level 6C. The other character that appears in the "Early Picross" stage is Mario, who appears in level 6H. Like their first appearance, Super Mario Bros. 3, they appear under the name Boo Diddly.
Boos (misnamed "Big Boo" in the English manual) make appearances in the Virtual Boy game, Mario Clash. They are among the nine enemies. Some of these enemies include, Pokeys, Para-Goombas, and Snakes. Though Mario Clash is a new style of Mario Bros., Boos do not appear in Mario Bros., but they do appear in a remake. In this game, Boos' objectives are to defeat Mario, but Mario is able to attack a Boo from the side.
Super Mario RPG: Legend of the Seven Stars
Boos return during the events of Super Mario RPG: Legend of the Seven Stars. The English translation erroneously calls them "The Big Boo". In the Japanese version in fact they are just called Teresa. These Boos, like other Boos that appear in previous games, hide behind their arms when stared at. However, some of The Big Boos teleport around the room, attempting to scare Mario and his friends. The Big Boos live in Kero Sewers along with many other ghouls, including Hobgoblins and Shadows. Often, these three undead monsters will assist each other in battle. Big Boos have no physical defense, making them extremely weak enemies (even a Goomba has three defense points). However, The Big Boos make up for their lack of defense with high evasion, meaning that most attacks against a The Big Boo will miss. In battle, The Big Boos command powerful lighting magic, including the attacks Lightning Orb and Bolt. The Big Boos also know the special attack Scream, which cuts a party member's attack and defense in half. When not using magic, a The Big Boo will approach one of Mario's party members and make a face, damaging that character. In this game there is also a stronger Boo variation called Li'l Boos, which are the same size as normal Boos but have blue tongues, similar to the first appearance of King Boo. The only true appearance of a Big Boo in this game is the one used in Bowser's Terrorize attack . One Big Boo lives in Monstro Town as part of the Three Musty Fears.
Boos make appearances in some of the Super Mario-Kun comics. A Boo appears on the cover of the sixteenth issue. This issue is based on Super Mario 64. Alongside it, Eyerok, a Bob-omb, and Bowser can be seen. Throughout the comic, they appear, too. Their role in the comic is similar to their role in the game, where they are enemies to Mario and attempt to attack him. Lady Bow, a notable boo, appears in a couple of the Paper Mario based issues. These issues are 25 and 27. Lady Bow appears on the cover of the twenty-fifth issue along with many of Mario's other partners, including Goombario, Kooper, Eldstar, and Twink. Besides appearing on the cover, she appears throughout the comic. Her role in the comic mimics her role in the game. Lady Boo reappears on the cover of the twenty-seventh issue and in the comic. Like in issue 25, Lady Bow's role in the comic is similar to her appearance in the game, where she helps Mario. In a Super Mario Galaxy based issue, Boo Mario appears on the cover along with more of Mario's transformations.
Game & Watch Gallery series
Game & Watch Gallery 2
Boos (called 'Boo Buddies' in the Note Board) appear in the modern version of Vermin in Game & Watch Gallery 2 When Yoshi looks in the general direction of them, they refuse to move. When Yoshi looks away, they move towards the eggs. They only appear in groups of two or just one.
Game & Watch Gallery 4
A Boo appears in the Modern version of Donkey Kong 3 and Mario's Cement Factory in Game & Watch Gallery 4. In Donkey Kong 3, if Mario or Donkey Kong turn their backs, it'll slowly come closer to the player that it's closest to. If it gets close enough, the Boo will either scare Donkey Kong and award the player points (1 player only), or scare Mario and give the player a miss. In Mario's Cement Factory, a Boo will fly into the two containers on the second floor, clogging them up. The player does not receive any points for removing the Boo.
Mario Party series
Boos make minor appearances in Mario Party. Their voices can be mimicked by one of the items: the Talking Parrot. Boos also appear in each of the game's board maps, excluding Peach's Birthday Cake. On the boards, they steal Coins or Stars from opponents, and they are willing to steal a few coins for free from any player, although they ask for 50 coins to steal a Star. If the player tells Boo that they don't want to steal anything, Boo will say Don't feel so guilty. Ha ha ha! Additionally, after the Luigi's Engine Room board is completed, a Boo, Koopa, and a Toad, along with the two runner-ups, surround the victor and congratulate them. Boos also appear as one of the shapes the player can cut in the minigame, Crazy Cutter.
Mario Party 2
Boos make several appearances in Mario Party 2. In it, they steal Coins or Stars as they did in the first game. However, Boo charges 5 coins to steal coins instead of a free steal; and steals much more than in the first game. Additionally, an item called the Boo Bell appears. When a player obtains it, they can ring for a Boo to come. The Boo that comes to the player's side is asked to steal Coins or Stars, depending on how much the player can afford. If the Boo is sent to steal Coins from an opponent, the opponent can fight it off by pressing the A button quickly. Additionally, the Boo can only steal a maximum of 30 coins or a maximum of one Star.
A Boo also appears in the battle minigame, Day at the Races, along with the others racers, Whomp, Thwomp, and Bob-omb. The Boo can disappear and then reappear in a further spot on the racetrack. However, it gets its face stuck in the ground sometimes, causing it to stop for a short period of time.
Mario Party 3
Boos also appear in Mario Party 3. Like in their previous appearances, they are not major characters in the game. They are among the Duel Partners in the Duel Mode Partners. It is Yoshi's default partner, and when an opponent attacks, Boo can counterattack for equal damage. Boo's attack is two, its health its one, and its salary is three coins.
Boos also appear in the unlockable party board, Waluigi's Island. Whenever a player runs into a Boo, it shows them to two paths. One of the paths is rigged. If the player chooses the rigged path, they are sent back to the start. If they choose the correct path, they continue on the board. In addition, Boos appear on all of the other boards, Chilly Waters, Deep Bloober Sea, Spiny Desert, Woody Woods, and Creepy Cavern, serving the same function as in the previous games. On the board map, a picture of them can be seen next to a grave. They also return with the Boo Bell. Boos are also in the minigames Curtain Call, Stacked Deck, Three Door Monty, and Crowd Cover. Two portraits of a Boo can also be seen in Locked Out.
Mario Party 4
In Mario Party 4, a Boo plays a major role as the host for Boo's Haunted Bash. This Boo has a whole new look with a wizard's cape. Like the other hosts in the game, it is the character to reward to player with a Star. Multiple Boos also appear on this board as background characters. One controls the Item Shop. Another one owns the Mystery Train, but it only gives out rides to the Boneyard if a Red Boo is not in its presence. Two side-games involving Boos requires a Mini Mushroom. One of them has the player pressing one of the keys of the Horn of Plenty to get one of three randomly chosen items. The second one has the character on a piano. They have to press the correct key at the right time until the end of the side-game to get a certain amount of Coins. A question mark space activates a Boo clock. If three people land on them, Big Boo will appear. Big Boo is basically an upgraded version of the Boo House; he targets all players instead of one. The caped Boo is also the fourth host of the Reversal of Fortune, who oversees what is given.
Boos also appear on every other board in the game, stealing coins and Stars for the player at a shop called the Boo House in the same manner as Boos' previous appearances in the Mario Party series.
Also, a Boo is an unlockable, playable character in the Extra minigame, Beach Volley Folley.
In Long Claw of the Law, a Boo is one of the possible characters that needs to be collected.
Mario Party 5
Mario Party 5 marks the first game in the Mario Party series where a Boo is a playable character in Party Mode. It is one of the new characters, the others being Koopa Kid and Toad. Boo's partner in the game is Koopa Kid. Since Boo is playable, it does not make any major background appearances.
Mario Party 6
A Boo reappears as playable character in Mario Party 6. In the game, there is also an orb called Boo Away Orb, which protects the player from a Red Boo. Red Boos also appear in the minigame Boo'd Off the Stage and Boonanza!. Red Boos also appear two boards, Towering Treetop and Castaway Bay. This game also marks the third appearance of Red Boos in the Mario Party series.
Mario Party Advance
They also appear in the Boo Cemetery where they are main characters in the quest, True-Blue Boo. This is a mystery quest, in which their golden Boo statue has been stolen, and they suspect that one of them has stolen it. They then notice that there are four of them, and it is up to the player to discover which one is a fake. Once he is discovered, the fake Boo reveals himself as Koopa Kid, and flees the area, leaving the statue behind. The Boos then reward the player with the Gaddget Predictotron.
They also appear in the Horror Condo in the quest Condo of Mystery!. A Snifit states that the condo isn't haunted, and the player has to head to the Boos and ask them to haunt it. After arriving at the cemetery, the Boos state that they actually forgot to haunt the condo, and head there to do so.
The end credits state that they haunt the condo daily.
Mario Party 7
Mario Party 7 marks the third Mario Party game where Boo is playable. It also marks Dry Bones' first playable appearance, who is Boo's partner throughout the game. Red Boos appear in the game, too. Red Boos appear in a few minigames such as Ghost in the Hall. In this minigame, Red Boos try to scare the players. Oddly, the Red Boos also try to scare Boo, if Boo is one of the characters. Boo shares the same special orb with Dry Bones, the Magic Orb. It allows them to double their dice roll during the next two turns and turns them invisible, making them immune to traps.
Mario Party 8
In Mario Party 8, Boo is a playable character for the fourth time in the Mario Party series. Boo's partner is Dry Bones, again. Red Boos appear in some minigames such as Boo-ting Gallery. Mario Party 8 also marks the first appearance of King Boo, who is the board host of King Boo's Haunted Hideaway. In order to get a Star, the player must find the room King Boo is hiding in and give him ten coins. There are three rooms, two of which contain a trap hole. If the player finds one of these rooms, several Red Boos will throw them in, forcing the player to start back at the beginning. When the player gets a Star, King Boo will rearrange the mansion's rooms. Red Boos appear as owners of the item shops.
Mario Party DS
Boo is not a playable character in Mario Party DS and plays no part in board play, but appears as an unlockable figure. They also appear in the mini-games Boo Tag, and Peek-a-Boo. In Boo Tag, a Boo tags a player. The tagged player must try to get rid of the Boo by running into another player. The player that spent the least amount of time with the Boo wins. In Peek-a-Boo, the players must find the wanted Boo. A lot of different Boos appear in outfits, such as hats. Several of the wanted Boo appear, and once they are all gone, the game is over. The team that won the most Boos wins.
Mario Party 9
Boo does not return as a playable character in Mario Party 9, however King Boo is the boss of Boo's Horror Castle. Normal Boos also appear on the board. When the player passes a Boo portrait, a Boo will emerge and chase after the player. These Boos will move three spaces after each turn, and if a Boo catches up to the players (or vice versa), the current captain will lose half of all of their Mini Stars.
During King Boo's Puzzle Attack, whenever King Boo's health is depleted halfway, he sends Boos to attack the player. If the player's cursor touches a Boo, they are temporarily stunned. King Boo only sends one Boo at a time.
Mario Party: Island Tour
Boo returns as a playable character in Mario Party: Island Tour. This is the fifth time that Boo is playable, the first time he is playable on a handheld Mario Party game, and he is the first character in the series to be removed as a playable character in a main installment of the series and then return in a later installment.
Mario Party 10
Boo reappears in Mario Party 10 as a non-playable character. Whenever the player reaches a certain spot in the board Haunted Trail, Boo appears and steals five Mini Stars from the current captain. Boo will continue to harass the player until passed off to another players or driven towards a streetlight. Boos also appear in the minigame Boo Burglars, as enemies the players must examine. Several Boos also make cameos in the Luigi Board.
Mario Golf series
Mario Golf: Toadstool Tour
Boo is one of the unlockable characters in Mario Golf: Toadstool Tour, the others being Bowser Jr., Shadow Mario, and Petey Piranha. Boos also appear in one of the courses, Bowser Badlands, where if the ball is hit through them they will affect its trajectory. Boos once again reprise their role of showing the wind direction, although they have another role; if a player delays in starting their swing, then two Boos carrying a sign will float across the screen. These signs describe basic gameplay tips, but some instead are taunts towards the player.
Mario Golf: World Tour
Boo returns in Mario Golf: World Tour as a playable character, this time being available from the start. Boos also appear in Hole 3 of Bowser's Castle floating in the sky alongside King Boo. For this installment, a Boo does not represent the wind.
Mario Tennis series
Mario Tennis (Nintendo 64)
A Boo appears in Mario Tennis for the Nintendo 64. It is classified as a tricky character, the only other character of this class being Paratroopa. It is also a default character. Most of the playable characters have courts named after them. However, Boo does not have its own court.
Mario Tennis (Game Boy Color)
Mario Power Tennis
Boo also appears in Mario Power Tennis where it is a playable character. When it wins a championship, Mario and Luigi would come out with the trophy and Boo would be missing. They look around and then the trophy would start to float. Boo then appears under it and Mario and Luigi are relieved. Then they start floating and the air is full of Boos who start flying around. Then the main Boo flies up to the screen and holds the trophy. Boo's taunt is to wave its racket while laughing. Boo's offensive power shot is the Boo Blast and its defensive power shot is the Boo Brigade Return.
Mario Tennis Open
Boo reappears in Mario Tennis Open, where he is again a playable and starting participant. He is one of the few tricky characters.
Paper Mario series
Boos appear in Paper Mario as friendly but tricky characters. White Boos live in Boo's Mansion, and yellow Boos live at Gusty Gulch. Their princess is Lady Bow, who lives in Boo's Mansion along with Bootler.
At first, Boos were being eaten by Tubba Blubba. Some Boos tried attacking the monster to save their friends, but they failed because of Tubba Blubba's "Invincibility". Seeking help from Mario, they captured Skolar the Star Spirit Mario was trying to save.
In the game, when Mario arrives at Boo's Mansion, Bootler tells him to get to the third floor. Boos have prepared some puzzle for him under the order of Lady Bow to test him. One puzzle is for Boos to form a rotary circle, throw an item several times and ask Mario to guess who is holding the item. The first time he opens a door a big Boo appears on the screen. In the eastern room of the second floor Boos holding a Record play this trick. In the western room of the second floor a Boo is guarding a chest box with a Weight in it. Mario plays the Record to lure the Boo away, and uses the Weight on the droplight of the mansion to open the western door on the first floor. Boos in the basement hide in a big chest box in the basement. When Mario opens the chest box, they play the puzzle with Super Boots. After Mario gets them, he is able to get Boo's Portrait with which he is able to get to the third floor. There, Lady Bow asks Mario to help the Boos to defeat Tubba Blubba with Skolar as a lure, and joins Mario's party to assist him.
In addition, a Boo appears after Mario got the Super Boots and tells him how to get the Star Pieces underground. And some Boos are flying in the sky who can be seen out of the windows. When Mario is about to set off, two white Boos appears to open the gate to Gusty Gulch under the command of Bow.
In the basement of Tubba Blubba's Castle, a yellow Boo tells Mario not to fight Tubba Blubba until he finds his weak point. According to his word, he sneaked into the castle to look for the weak point. After Mario gets Yakkey, 3x2 peasant Boos appear in front of Tubba Blubba's Castle and, seeing Mario and Bow get out and Tubba Blubba chasing, help the co by blocking the door.
After Tubba Blubba is defeated, Boos at Gusty Gulch begins to talk about a female Boo called Victoria. A Boo who wants to marry her will ask Mario, if Mario goes there, whether he should give her a Fire Flower or a Volt Shroom as a present. Whatever Mario's answer is, the Boo will finally win Victoria's love, and they plan to get married when Lady Bow comes back.
After Mario defeats Bowser, two Boos come to Princess Peach's Castle to see the party, one white and one yellow. The white one says he comes to see Lady Bow in her finery. He thinks that Bow is prettier than Princess Peach. The yellow one says he's going to have a wedding ceremony after the party. In the end-of-game parade, three white Boos along with Bootler are carrying Tubba Blubba's dead body.
Paper Mario: The Thousand-Year Door
In the GameCube game Paper Mario: The Thousand-Year Door, the Boos live in a tower called Creepy Steeple. When Mario first enters the steeple there is only one Boo in the room who says that he misses his friends. He will let Mario through one door (Blocking him from using any other doors), where he finds a chest. When he opens it, 200 Boos fly out. The last Boo asks Mario "How many Boos came out of that chest?" If Mario answers correctly the Boo will give him an Ultra Shroom. If Mario is wrong the Boo will give him one Coin. Upon returning the main room, the first Boo is still there. He asks if Mario is going to be mean to him. If Mario answers "No", a bunch of Boos (The same ones from the treasure chest) will start flying around the main room, attempting to swarm Mario. If successful, they will throw Mario out of the Steeple, exactly like the Sentinels in Tubba Blubba's Castle. If Mario Spin Attacks them several times, they merge into the Atomic Boo. Once the Atomic Boo is defeated, Mario can fight Boos in other parts of the Steeple. The original Creepy Steeple Boo sends a letter to Mario that thanks him for freeing his friends, despite the fact that he and his friends attacked him. He signs the letter "The Creepy Steeple Boo". More Boos can be found in the Pit of 100 Trials. In this game, Boos attack by becoming invisible and getting close to Mario or the partner, then becoming visible and making a glare at them, causing 3 damage. However if Mario or his allies gets lucky in battle, the Boo who tried to attack Mario and his partner will be tired out. The game also introduces a more powerful sub-species, the Dark Boo, which can be found in Poshley Sanctum.
Super Paper Mario
Boos return as enemies in Super Paper Mario. In this game, they turn invisible if Mario looks at them, but they turn visible and chase him if he is not looking at them, similar to their behaviour in Super Mario 64. To defeat them, the player must approach them from the back and then attack them. Additionally, the Flipside Arcade houses a Boo themed shooting game, Mansion Patrol. This game has the player shooting Boos (Both Normal, Dark and occasionally Atomic Boos) to score points. A Toad frequently appears to give power ups, as long as the player saves him from his Boo captor.
Paper Mario: Sticker Star
Boos reappear in the Nintendo 3DS title, Paper Mario: Sticker Star. Like most enemies, the Boos' appearance is more modern compared to the other appearances of the past three installments. They only appear in World 4-3: The Enigmansion. Mario must use the Book of Sealing, given by a Toad, to seal away every Boo that has escaped. In battle, Boos can make themselves disappear and can slap Mario or they can go up to Mario and scare him by making a scary face. They are able to make Mario dizzy in this game with their scare attack preventing him from using any of his defensive/dodging stickers. The attack is determined by the sound the Boo makes when it disappears.
Among the normal Boos, there is one golden Boo that drops several Coins after Mario defeats it, which can be found in the safe. This Boo attacks exactly like the normal ones. Another notable Boo is one that is crumpled, which is found in a trash can. In battle, it only has 1 HP and cannot attack. Additionally, a group of eighty-two Boos called a Boo Stack also appears. Once all the Boos get returned to the book and it is placed in its rightful spot, Kamek appears and turns all the captured Boos into the mini boss, Big Boo. After defeating the mini-boss, the Boos are put to rest once again and the mansion is restored to its former state.
In Luigi's Mansion, Boos are the main antagonists. King Boo sends a letter to Luigi, stating that he had won a mansion in a contest. However, Luigi did not enter any contest, but pays no attention, and heads off. Mario goes on ahead, but get captures by the King of the Boos, King Boo. Luigi has to rescue him from the Boos, using E. Gadd's Poltergust 3000. Luigi manages to find his brother, stuck in a painting. He ends up having to battle King Boo in order to rescue Mario. King Boo sends them to the roof of the mansion, and uses a Bowser suit to battle him. However, the suit's head could come off once a spikeball was thrown at him, and King Boo would float out, leaving him open for Luigi's Poltergust 3000. After taking away all of his 500 HP, King Boo is turned into a painting, and Mario is restored back to his normal self. However, King Boo manages to escape by unknown means and reappears in several future Mario games.
There are 50 Boos (51 including King Boo) which Luigi must suck up in the Poltergust 3000. Of these, 15 of them make up the portrait ghost Boolossus, with King Boo as the final Boo. Unlike the other ghosts in the game, Boos will appear in a room, even when the lights are on. When Luigi attacks them, they usually run away, but occasionally twirl into him to cause damage. All of the 36 Boos, excluding King Boo and counting Boolossus as one Boo, have puns in their names.
The Boos in Luigi's Mansion seem to be the reverse of the Boos in the Super Mario series: while the Boos in Super Mario sneak up on Luigi when he's not looking and then disappear/hide when he looks at them, the Boos in Luigi's Mansion (excluding King Boo and Boolossus) hide when Luigi's not looking at them and then they face Luigi when he finds them. One of the Boos, Peekaboo, references the fact that Boos are notorious for hiding.
Mario & Luigi series
Mario & Luigi: Superstar Saga
In Mario & Luigi: Superstar Saga, Boos are enemies that only appear in Bowser's Castle; they aid Bowletta along with the rest of the minions who believe her to be Bowser. Outside of battle, they hover around, attacking Mario and Luigi when they are not looking. However, they look away if Mario or Luigi make eye contact. The Boos also show the ability to teleport. In battle, a Boo has two attacks. For the first attack, the Boo will teleport in front of one of the brothers and attempt to bite him. For the second attack, it will fly up into the air and swoop into one of the Mario Bros., which can even prohibit the use of certain moves or items. The former attack can be countered with Mario or Luigi's hammer, whereas the latter can be dodged or countered by jumping. In either of these attacks, pay attention to whether or not the Boo initially makes a silly face. If it does, it will target Luigi; if it does not, it will target Mario. Boos are weak to Firehand and Fire Bros. They also have low HP, but a very high defense rating.
Boos wearing mining helmets and brandishing pickaxes also appear in levels 4 and 5 of Mysterious Mine Carts. If these Boos touch Mario, they will cause the loss of a diamond. They will slowly shrink until they disappear if the flashlight is pointed at them.
Mario & Luigi: Partners in Time
During the events of Mario & Luigi: Partners in Time, Boos appear in Bowser's Castle of the past. Mario, Luigi, Baby Mario, and Baby Luigi meet a lot of them while trying to reach the end of Baby Bowser's Castle. In the field, Boos attack the same way as they did in Mario & Luigi: Superstar Saga, but in-battle they fight differently, and have significantly decreased defense. They cover their faces, but when they start to attack, the Boo would take one of its hands off one of its eyes, showing who it is about to attack. If it shows its right eye, it will attack Mario, but if it shows its left eye, it is attacking Luigi. Then it would appear from behind, and charge towards one of the Mario Bros.. However, this could easily be countered with a jump. If the attack was not countered then the boo would steal an item which could only be recovered by defeating the boo. After a turn for each brother, the boo would Flee with the item. They also appear during the battle with Sunnycide. After the Mario Bros. flip the egg monster over, Sunnycide could summon a Boo to attack Mario or Luigi. Dark Boos also return in the game, and are found in the Gritzy Caves. Also, when a Handfake has a Bowser sign, a Boo or a Fly Guy carries a flame from Bowser's mouth.
Mario & Luigi: Bowser's Inside Story
Boos make a reappearance in Mario & Luigi: Bowser's Inside Story. They appear in Bowser's Castle, after being brainwashed by Fawful, and they also help many of the former minions of Bowser push the Fawful statue against the powered-up Bowser. In addition, a Boo appears whenever Bowser gets a level up. The Boo (just like the Lakitu in Mario's or Luigi's level up screen) holds the circle for getting a bonus upgrade for the stat of the players choice.
An odd thing shown in the game is a Boo and Koopa Troopa in love with each other in Bowser's Castle. They not only love each other, but also love Fawful, due to the brainwashing that was put on the minions.
One of the Boos in Bowser's Castle holds a plate with Midbus's emblem on it.
Mario vs. Donkey Kong
A Boo appears in Mario vs. Donkey Kong. Throughout the whole game, they make only one appearance in the Spooky House. It uses its regular battle strategy, which is attacking Mario when his back is turned and looking away when Mario turns around. The Boo sticks its tongue out at Mario if he is about to lose a life.
Mario Pinball Land
In Mario Pinball Land, Boos appear in certain areas in the game. Mario cannot hit them at the front because they will become transparent as Mario passes through them. If Mario hits one of the Boos on its back, the Boo disappears and it is worth two-thousand points. They occasionally will disappear and reappear to another spot in the area.
A Big Boo also appears as one of the game's bosses. He is the boss of the haunted house and can only be damaged by hitting it from behind.
Boos appear in the Japan-only Nintendo DS game, Yakuman DS. They are one of the many opponents. Other opponents include, Goomba, Diddy Kong, and Hammer Bro. Boos are not unlockable or playable. They are opponents from the start of the game.
Dance Dance Revolution: Mario Mix
Some Boos make appearances in Dance Dance Revolution: Mario Mix in both regular and giant varieties. Their objective is to try to make Mario miss a step. If successful, they block Mario's next move. Mario is able to step on regular Boos to hold them back, but he cannot do the same to their gigantic counterparts.
Mario Baseball series
Boo makes its first baseball appearance in Mario Superstar Baseball. It is on Wario's team as a starter. Boo is tied with Peach with its pitching, losing only to Bowser and Waluigi. Its special running and fielding move is a Super Jump. The only worse fielder in the game is Bowser.
Boo makes another appearance in Mario Super Sluggers. It is on Wario's team once again, and it can be unlocked in Wario City by finding it in between boxes and completing its challenge. Like its predecessor, Boo has the highest ranked pitch in the game, along with Peach (9/10), but its other stats range from average to low. Boo displays good chemistry with King Boo, Shy Guy, Magikoopa, and Blooper. Its chemistry with Luigi, Baby Mario, and Baby Luigi is bad.
Also, a new item, called Mini Boos, appears. When activated, Mini Boos turn the ball invisible for a short period of time. If used against a computer, the player stops moving until the Boos go away. If used when a special hit is effect, the Boos do nothing.
Super Princess Peach
Boos appear in Princess Peach's first starring game, Super Princess Peach. They act as normal Boos, hiding their face when Peach looked, and attacking when Peach has her back turned. However, there is an angry variety called the Mad Boo, which attacks the opposite way. They attack when Peach looks at them, and gander around for her when she looks away. Big Boos and their vibe counterparts, a new species called Mad Big Boos, also appear.
King Boo also appears, as one of the bosses of the game. He will attack Peach by shooting fireballs at her, but she can use these to light the candles on the wall, damaging him. His vibe during the game is Gloom, which he will use to douse the candles.
There are several sections of the game where a mass of blindfolded Boos will charge towards Princess Peach.
Mario and the Incredible Rescue
Boos make an appearance in the chapter book, Mario and the Incredible Rescue. The cover features two Boos. One of the Boos is from Mario Kart: Super Circuit and the other is a mirrored Boo from Luigi's Mansion. They are known as "ghosts" in the book. Boos capture Princess Peach and take her to Bowser's Castle. Mario also sees a vision of Princess Peach being caged and surrounded by six Boos. Boos are eventually defeated by Mario at the end of the book. To defeat them, Mario casts a spell on them and sends them flying out of the castle.
Boos also appear in the Nintendo DS game, Tetris DS. They are one of the few multiplayer items. Boos are able to block the opponent's sight of the upcoming block, making it harder for the opposing player to see the upcoming block. They do not appear in single player.
Mario Hoops 3-on-3
A Boo makes an appearance in Mario Hoops 3-on-3. It appears as an unlockable character. It is unlocked by beating the Flower Tourney on hard. Boo's special ability is the Thriller Dunk. This is performed by tapping a B-like shape twice. In the game, the Boo is classified as a tricky character. The other tricky characters are Paratroopa, Fly Guy, and Moogle. Boo is also one of many of the unlockable characters to not have a baller name.
Mario Strikers Charged
Although Boos do not appear in Super Mario Strikers, they do appear in its sequel, Mario Strikers Charged. Boos, along with Hammer Bros., Toads, Koopa Troopas, Birdos, Dry Bones, Monty Moles, and Shy Guys, are sidekicks to the many captains. They appear with a helmet with the logo of their team captain. Boos are classified as a playmaker. The only other playmaker sidekicks are Toads. Their skillshot is Possess. The given description of the Boos is: "This ghost can fly up and down the sidelines with great speed, using precise and accurate passing to lead his team to victory. His long-distance shots are quite weak though and his hits are not very powerful. If Boo gets a chance, he can Possess the ball and score at will." Boo's appearance in this game resembles that of the Spooky Speedster.
Mario & Sonic series
Boos are minor, supporting characters in Mario & Sonic at the Olympic Games. It appears in the version for the Wii and Nintendo DS. They are not playable characters, but they can be seen in the crowd with other Mario enemies, including Bullet Bills, Goombas, and Bob-ombs. Though Boos do not appear in Mario & Sonic at the Olympic Winter Games, King Boo is one of the rivals. A Boo teams up with King Boo in the Nintendo 3DS version of Mario & Sonic at the London 2012 Olympic Games (in Story Mode) to face off against Sonic the Hedgehog and Miles "Tails" Prower in Badminton-Doubles.
Super Smash Bros. Brawl
Mario Sports Mix
Boos appear in Mario Sports Mix. They appear as the only members of the crowd in Luigi's Mansion circling around the court. Their appearances are in Basketball and Volleyball matches and their laughs can also be heard during the game. Mainly, they can be heard when the player gets a point.
Luigi's Mansion: Dark MoonLuigi's Mansion: Dark Moon. They have names that play on "Boo" as in the first game. King Boo appears, having a similar appearance to that of his debut. Regular missions in the game usually have a Boo in an object that is invisible, and Luigi must find it by revealing the invisible object with the Dark-Light function of the Poltergust 5000; the Boo appears after the revealed Spirit Balls have been vacuumed up. However, not all Boos are hidden in invisible objects. To capture a Boo, Luigi must expose it with the Dark-Light, vacuum and pull on the Boo's tongue, and release it with the Button, damaging the Boo as it bounces around the room. When its HP reaches 0, it can be sucked up. After it is sucked up, it will be stuck onto the nozzle. Luigi then presses the Poltergust 5000 against the ground to shove it inside. When Luigi returns to the Bunker after completing a mission, any captured Boos are sent to the Boo Canister in the Vault. After all Boos in a mansion have been captured, a bonus mission is unlocked for the corresponding mansion. There are a total of 32 Boos, 33 if counting King Boo.
Captain Toad: Treasure Tracker
Boos return in Captain Toad: Treasure Tracker, functioning the same as they did in Super Mario 3D World with a few minor tweaks. Despite still chasing the player when not watched, Boos and Big Boos can easily be destroyed with Toad's or Toadette's headlamp.
Super Mario Maker
Though their specifics change from game to game, Boos are usually bashful, cowardly, spherical ghosts. They have black eyes and fangs. Although they are typically white, there are sub-species of the Boo race which appear in different colors, such as pink. From Luigi's Mansion onward, it became common to picture Boos sticking out their tongues. Since Super Mario 64, Boos have been portrayed with a unique, high-pitched laugh. This laugh is the same as Bowser's, sped up several times. However, Boos got a new laugh starting in Mario Power Tennis, being more of a haunting laugh than before. Although most species of Boo will cover their faces and become intangible when looked at, they will often sneak behind their victim and strike when their back is turned. Due to their ghostly abilities, Boos are generally invulnerable and cannot be harmed by ordinary means of attack. Despite this, they are not invincible and are especially vulnerable to the light. If they are exposed to a source of light, Boos are usually weakened, and in some cases they may even be defeated. Usually, they can also have other weaknesses. Because of their intolerance to the light, Boos will often hide if they are trapped in lit rooms. While the ghosts generally appear to be more afraid of their victims than their victim is of them, this is not always the case. Some Boos have absolutely no problem meeting their victim's gaze, and will even try to attack. Boos tend to have a taste for the fine arts, as seen in Paper Mario and Mario Party 4. In Luigi's Mansion: Dark Moon it is shown that Boos will actually save someone if only later to trick them. This is seen when Luigi watches through pipe in the Old Clockworks as a Yellow Toad balances on a ledge. Some Boos appear and watch the Toad before it falls off the ledge, which is later found to be quite a drop, the Boos show a large amount of concern and shock when this happens before diving down for the Toad. The Toad though is later found in a painting because of the Boos.
Boos are often characterized by their sub-species despite their similarities in appearance and attack manner. Boo species tend to wildly differ in size, with some such as Boo Buddies being miniature, while others such as the Big Boos boast gigantic size.
In Luigi's Mansion, Professor E. Gadd notes that Boos have mysterious powers that increase when they gather together in large groups. While it is difficult to stop them, they can be weakened if one is able to defeat members of the group. Although in Luigi's Mansion: Dark Moon this does not seem to affect them or King Boo in any way suggesting that they do not always need to be in groups anymore.
Boos have few predators, their only known threat being a Clubba named Tubba Blubba. This is first seen in Paper Mario, when he began devouring Boos; this, however, may have been his revenge for the countless times the ghosts scared him.
Boos typically make their homes in dark forests, such as the Forever Forest and Boo Woods, and tend to be most commonly found in older buildings such as mansions, castles, and Ghost Houses. Although most of them live in such dwellings, they also seem to enjoy wastelands. One of the only known Boo villages lies in Gusty Gulch.
They are typically found in groups because their fears make them find strength in big numbers.
Though most Boos tend to side with King Boo, they seem to, for the most part, actually stand as neutral. For example, Boos are by now a common enemy in most side-scrolling games; however, in the Paper Mario series and now increasingly in the multiplayer games, Boos have been seen to take the side of the Mushroom Kingdom. Lady Bow, in fact, helped Mario save the kingdom in Paper Mario. In another example, the Boos in the Creepy Steeple from Paper Mario: The Thousand-Year Door attack Mario initially, but later in the game Mario is sent a friendly letter from one of them. King Boo, on the other hand, has several times been shown to wish nothing but terrible things for Luigi (but this could be accredited to Luigi being easily scared, making him a prime target for most Boo antics). They also have attacked Bowser in some games, such as the Mario Party series. (5-8)
As hinted in the game Super Mario Galaxy, Boos have their own unique language called Booish. It most probably is spoken only by Boos since Mario cannot understand it. However, upon absorbing a Boo Mushroom and turning into Boo Mario, he gains the ability to read it, if not speak it as well.
Paper Mario also features Boos replacing certain words with "Boo", possibly as a part of the Booish language:
Official profiles and statistics
Super Smash Bros. Melee
Mario Superstar Baseball information
Names in other languages