King Boo

"I am not afraid of you, fool! I fear only that infernal Poltergust 3000 you carry on your back! Stupid machine! But I am a KING among Boos! I swear it... I shall fear no mere house-cleaning device!!"

- King Boo

King Boo is the leader of the Boos and ghosts, ruler of the Paranormal Dimension, the main antagonist of the Luigi's Mansion series, and the arch-enemy of Luigi. He is also an ally of Bowser, who has aided him in his various schemes. He is more than capable of devising and enacting villainous plans by himself, including his capture and imprisonment of Mario. Although not the biggest Boo, King Boo has abilities that far surpass that of the average ghost; he wields a number of impressive magical abilities, including the ability to materialize objects. King Boo's magical power is directly proportional to the number of Boos in his vicinity.

Luigi's Mansion
Prior to the events of Luigi's Mansion, Professor E. Gadd captured Boolossus. Using a device called a Ghost Portrificationizer, E. Gadd trapped Boolossus in a portrait and hung it in his Gallery inside his lab in Boo Woods. This angered King Boo, so he devised a plan to rescue Boolossus.

King Boo and his forces attacked the laboratory. The Boos reversed the settings of the Portificationizer and released Boolossus. King Boo then ordered that all the ghosts that E. Gadd had captured should be released as well. After releasing all the portrait ghosts, King Boo created a huge mansion and placed it next to the professor's lab. King Boo, his Boo minions, and the recently freed portrait ghosts decided to inhabit the mansion. With a huge mansion and an army of ghosts, King Boo decided to exact revenge upon Mario and Luigi for all the Boos they attacked during their adventures by capturing them.

Hoping to capture Luigi, King Boo sends him a letter. The message states that Luigi has won a mansion in a contest and provides the location of the prize. Luigi asks his brother Mario to investigate the mansion and says that he is arriving shortly thereafter. Mario travels to the mansion, but right after he enters, he is instantly overwhelmed by King Boo's forces and captured. King Boo then imprisons him in a portrait, similar to how Boolossus was imprisoned. Mario is placed in the Secret Altar in the basement of the mansion. With Mario captured, King Boo is ready to capture Luigi.

While waiting for Luigi, King Boo and his Boo minions wait behind a secret door in the floor of the Storage Room. When Luigi enters the room and reveals the secret door, the Boos spring out from the floor. However, the Boos (including King Boo) stop their surprise attack when they notice the Poltergust 3000 on Luigi's back. Luigi had been given the Poltergust 3000 by E. Gadd so he could recapture the Portrait Ghosts and save his brother. The Poltergust 3000 is one of the few things King Boo fears, and he and the other Boos scatter and hide in the mansion. King Boo travels to the Secret Altar in the mansion's basement.

The special key that Vincent Van Gore holds unlocks the path to the Secret Altar. However, he is locked in The Artist's Studio on the third floor of the mansion. After capturing him and retrieving the key, Luigi discovers that he still cannot enter the Secret Altar. If Luigi attempts to enter the Secret Altar without catching a certain number of Boos, King Boo blows Luigi to the Foyer, the entrance of the mansion. Since King Boo's power is related to the number of Boos in the mansion, Luigi can weaken him by capturing the Boos hidden in various rooms. After Luigi captures at least forty of the mansion's fifty Boos, King Boo loses his ability to blow Luigi from the Secret Altar. Once King Boo is weakened, Luigi enters King Boo's Secret Altar.

Upon entering the Altar, Luigi notices King Boo staring at Mario in the painting while insulting the brothers' intelligence for having fallen for his scheme. King Boo then turns around to face Luigi and states his desire to add a Luigi painting to the collection. After this, King Boo flies into the Mario painting hanging on the wall. The picture of Mario transforms into Bowser, then the new Bowser painting inhales Luigi into the canvas, transporting Luigi into an arena that resembles the mansion's roof. Luigi then confronts Bowser. In reality, this is a suit of Bowser that King Boo controls from the inside and uses as a shield.

King Boo uses this Bowser suit to attack Luigi, shooting flames at him causing ten HP of damage, and even attempting to inhale him into its mouth, deducting fifteen HP upon spitting him out. The suit also has the ability to create three large spiked bombs and toss them at Luigi, which when exploding, take away ten HP. Luigi can use his Poltergust 3000 to grab one of the spiked explosives and release it into the Bowser suit's mouth. This attack blasts the head off of the Bowser suit, causing King Boo to exit the shield and expose himself to the Poltergust 3000. Luigi begins to deplete the ghost's five hundred hit points. Meanwhile, the suit's head hovers around the arena, shooting icy blasts at Luigi, which freezes him briefly. King Boo, after a while, flies back into the dormant Bowser body and the head reattaches itself, resuming the fight. Luigi repeats this process, slowly weakening King Boo, but after King Boo's life falls below two hundred HP, the head of the suit reattaches backwards whenever he returns to the Bowser body, causing the Bowser suit to run around aimlessly. The suit soon corrects its head and resumes the fight. Eventually, Luigi manages to completely drain King Boo's HP and capture him inside the Poltergust 3000.

After the battle, Luigi receives King Boo's crown, worth 5000G, and adds it to his treasure collection. Upon returning to the mansion, Luigi quickly travels to Professor E. Gadd's lab. In the lab, Luigi connects the Poltergust to the professor's Portrificationizer, dumping its contents into the machine. Both King Boo and the fake Bowser appear on the Portificationizer portrait. With King Boo captured, Professor E. Gadd hangs his new portrait in an empty room in his gallery.

Luigi's Mansion: Dark Moon
King Boo reappears in the Nintendo 3DS game, Luigi's Mansion: Dark Moon once again as the main antagonist and final boss. He looks similar to his original appearance from Luigi's Mansion, but retains the full set of teeth and sped-up voice he had in his appearances in the other Mario titles and is also much larger. He has a purple tongue and mouth and glowing purple eyes, and the dark area around his eyes is bigger and darker.

It is revealed later in the game that Professor E. Gadd had sold the portrait of King Boo from Luigi's Mansion at a garage sale (a car boot sale in British English versions), which is how the king escaped. King Boo shatters the Dark Moon into six pieces with a bolt of energy from his crown, which contained a power-enhancing gem. This turns the once-friendly ghosts of Evershade Valley hostile and mischievous as well as sprawling the Dark Moon's pieces to each Mansion of the valley, which Luigi has to recover.

Towards the end of the game, it is revealed that he has captured Mario like he did in the last game. After Luigi gets the last Dark Moon piece and returns to the Bunker, he captures Luigi and pulls him back into the Paranormal Dimension. He reveals to Luigi he escaped from the painting and broke the Dark Moon, causing the ghosts to become under his control. He intends to use the ghosts as an army to conquer the world while also planning to put Luigi, E. Gadd, and all their friends into paintings for revenge. He then engages into battle with Luigi but he is defeated and is sucked into the Poltergust once again.

Luigi's Mansion Arcade
King Boo appears as the final boss for Luigi's Mansion Arcade, leading his ghosts in an uprising to get revenge on Luigi and Professor E. Gadd. However, after Luigi has sucked in all of his minions with the Poltergust 5000, King Boo assaults Luigi on the terrace of the Treacherous Mansion. Protected by a magical barrier, he can only be attacked once he lets his guard down. King Boo throws bombs at the player, moves very quickly and clones himself multiple times over the course of the fight. However, only the original has the power-enhancing gem on his crown. Just like in Luigi's Mansion: Dark Moon, he escapes the Poltergust 5000 two times before being defeated for good.

Luigi's Mansion 3
King Boo reappears in Luigi's Mansion 3 with the same design as in Luigi's Mansion: Dark Moon, albeit with more detail on his crown and a more expressive scowl. Once again, King Boo is the main antagonist, having been released by Hellen Gravely, the owner of the hotel and a big fan of his, in order to impress him. Seeking revenge on Luigi for his previous defeats, he traps Professor E. Gadd, Mario, Princess Peach, and the three Toads in portraits before going after Luigi, who manages to escape his capture by going down a laundry chute. He later ends up entrusting Hellen Gravely with Mario's portrait, although it's implied that he intended for Luigi to win both due to Gravely's failures by that time and so he could capture both alongside Peach by using the latter as bait, also recapturing the Toads and Professor E. Gadd just after Mario was freed. However, he ultimately fails to capture Luigi atop the hotel due to Polterpup shoving him out of the way of King Boo's portrait, and then proceeds to battle Luigi. To defeat King Boo, Luigi must throw a bomb from a spiked ball King Boo throws into his mouth, then pull on his tongue with Gooigi's help and slam him repeatedly. Upon doing so, King Boo will then generate a clone that attacks alongside him. After this is done twice, King Boo will enlarge the painting, and Luigi will then have four minutes to attack King Boo one last time. The battle ends with King Boo being recaptured by Luigi and placed in a containment jar alongside Gravely, much to his chagrin.

The clones that King Boo summons in the final battle reference his original design from Luigi's Mansion, as they only have two teeth, lacking the middle two fangs King Boo is depicted with now.

If the player gets a game over at any point during this game, King Boo will be shown trapping Luigi in a portrait alongside the rest of his friends.

Unlike in Luigi's Mansion and Luigi's Mansion: Dark Moon, once King Boo is captured, the player cannot save their data after the credits have been played. This make it impossible to save the Coins, Bills, and Gold Bars collected before the final battle; the player must load up a story checkpoint before entering the door to the final battle.

Super Mario-Kun
King Boo appears in a short chapter on Luigi's Mansion of volume 26 of Super Mario-Kun. As in the game, Luigi reveals King Boo by knocking off the Bowser illusion's head. Unlike in the games, however, King Boo is immediately sucked in.

Super Mario Sunshine
In Super Mario Sunshine, a different "King Boo" is the leader of the Boos on Isle Delfino, and appears as a boss in Sirena Beach during its fifth episode, King Boo Down Below. King Boo dislikes spicy food, which serves as the key to defeat him. Based on his defeat animation, he is apparently made out of goop, meaning that Bowser Jr. might have drawn him.

This King Boo was given his name by the American localization team, but the original Japanese version assigns a unique name to this character (ボステレサ, meaning "Boss Boo").

Super Mario 64 DS
King Boo appears in the Super Mario 64 remake, Super Mario 64 DS. As the boss who locked Luigi away, he captures a part of Peach's Castle. However, in the English translation of the game, King Boo is incorrectly called Big Boo, even though he is clearly different from the Big Boo of Big Boo's Haunt.

Regardless, King Boo aids Bowser in his conquest of Peach's Castle by capturing Luigi and imprisoning him in Big Boo's Haunt. Specifically, Luigi is imprisoned behind a painting of himself inside the haunted mansion. Mario heads to the Big Boo Battle level to save Luigi. Upon entering the level, he is greeted by King Boo, and from there he must listen for King Boo's cackle and enter the door where it is loudest. Eventually Mario reaches the end and fights King Boo to free Luigi. To defeat King Boo, Mario has to follow King Boo's movements in the arena's mirror and stomp him when he comes close enough. Ultimately, Mario saves his brother from King Boo and together (with Yoshi and Wario once he is saved) they defeat Bowser and save Princess Peach.

New Super Luigi U / New Super Mario Bros. U Deluxe
King Boo makes a small cameo in New Super Luigi U and New Super Mario Bros. U Deluxe in the Secret Exit of Vanishing Ghost House, where his art from Luigi's Mansion: Dark Moon can be seen in the form of a sign hanging from the side of the building.

Mario Kart: Double Dash!!
King Boo is an unlockable character in the racing game Mario Kart: Double Dash!!. This is his first appearance in a spin-off, and also his first playable appearance, resulting in further appearances in the Mario Kart series. Despite being a ghost, King Boo is classified as a heavyweight driver due to his large size. His partner is Petey Piranha, and both have the ability to use almost any other character's special item (if they are going to get a special item, it is randomly chosen, based on their race position). They cannot receive the Green Fireball or the Birdo's Egg, however.

King Boo's personal kart is the Boo Pipes, which is a gray version of Petey's Piranha Pipes kart, but with an image of a Boo instead of a Piranha Plant where the item-using partner stands. However, the more important difference between the two is that, while the Piranha Pipes has high speed and low acceleration, the Boo Pipes is vice versa, with low speed and high acceleration. The only other heavy kart like this is the Offroader in Mario Kart Wii.

When partnered with Petey Piranha, the camera is slightly zoomed out behind them during gameplay; this pairing is the only one to do so. He also appears to slightly shrink when he becomes the driver. On the second "Thanks for Playing!" screen (after beating the Special Cup with a Gold Trophy while all other cups already have a Gold Trophy), King Boo appears in the background chasing Wario and Waluigi.

Mario Kart DS
In Mario Kart DS, King Boo, along with six other bosses from Super Mario 64 DS, re-appears as one of the bosses of the Mission Mode. King Boo is the boss of the fourth level, with the playable character being Peach. Peach attempts to collect 50 coins on the Lava Arena within the time limit, but after a few seconds King Boo appears, surrounds her, and then attacks her, stealing ten of her coins and making all uncollected coins disappear, before teleporting to another area of the arena. To take them back, Peach has to drive and bump into King Boo, making him transparent. Then, she can drive through him again, retrieving her coins and making the other coins reappear. However, King Boo soon reappears a few moments later to get the coins again. After collecting all fifty coins, Peach finally defeats King Boo. Additionally, the Bowser and King Boo painting from Luigi's Mansion appears in the Luigi's Mansion race course, although only Bowser can be seen in the painting.

Mario Kart Arcade GP 2
King Boo makes a cameo appearance in Mario Kart Arcade GP 2 on the Yoshi Park 2 course in the Horror House. Here, a large picture of him appears near the end of the course where it inhales many ghosts and the racers. As the player drives into his mouth, they are warped back to the beginning of the course.

Mario Kart Wii
King Boo returns in Mario Kart Wii as a large-sized driver, and is unlocked by completing the Star Cup in 50cc. He is also unlocked by playing 750 races. He features a good handling bonus and a small off-road bonus, which comes in handy for vehicles like the Offroader, Wario Bike, and Phantom. He is used as the Staff Ghost on the SNES Ghost Valley 2 course. In the credits, he appears driving the Jetsetter on the SNES Ghost Valley 2 course.

King Boo is one of very few characters to have his standing animation on the Character Select screen altered from Mario Kart: Double Dash!! In this case, his tongue is not sticking out of his mouth the whole time like before; now, before selecting him, his tongue is still in his mouth. Other characters whose sprites were altered include: Yoshi, who is now looking at the screen; Birdo, whose bow was made larger; Diddy Kong, who is looking in a different direction; Wario, who now has short sleeves; Waluigi, who now performs a slightly different animation when selected; and Donkey Kong, whose mouth is not as wide open and is looking in another direction.

Mario Kart 8 Deluxe
King Boo returns as a playable character in Mario Kart 8 Deluxe, the enhanced port of Mario Kart 8 released for Nintendo Switch. He is among the lightest of the 5 heavyweight groups, sharing his stats with Rosalina and Link. He has the lowest top speed and weight of all the heavyweights, all while sharing similar acceleration as other heavy groups. However, his handling is amongst the best of the heavyweights (shared with the metal characters). He also has the strongest mini-turbo boost of the heavyweights. Finally, his traction stat is noticeably strong, being far and away the best in his weight class, and the 3rd best base traction stat in the game. He is also the 200cc Staff Ghost for Twisted Mansion.

Mario Kart Arcade GP DX
King Boo appears as a featherweight playable character in Mario Kart Arcade GP DX, sharing his stats with Baby Mario and Baby Peach. He was only playable during a limited-time event campaign held between October 18, 2017, and January 10, 2018, in the Japanese version, though Bana Passport users could unlock King Boo permanently when they collected three in-game stamps, making him usable even after the event's duration.

Mario Kart Tour
King Boo reappears as a playable character in Mario Kart Tour available from the October 23 Halloween Tour. He is classified as a Super character, and his Special Item is the Lucky Seven. An alternate playable version of King Boo, available as the Week 2 Spotlight racer from the same Halloween Tour, appears using his design from Luigi's Mansion 3. This marks the first time King Boo is playable using his appearance from the Luigi's Mansion series, and the first time he physically appears with this design outside of the Luigi's Mansion series. This variant is listed as a High-End character and has the Bob-omb Cannon Special Item.

In pre-release materials, including the beta test, King Boo was classified as a Super Rare character and his Special Item was the Double Bob-ombs.

Mario Superstar Baseball
King Boo is a playable character in Mario Superstar Baseball and a member of Wario's baseball team, the Wario Greats. He is a default character, meaning he does not need to be unlocked. He is a power hitter, using his white, crowned, Boo-like baseball bat to hit long outfield drives and even homeruns. King Boo has good chemistry with Boo and Petey Piranha, and he has bad chemistry with Luigi. However, like Boo, his hits are usually very curved, and this causes them to be fouls most of the time.

Mario Super Sluggers
King Boo reappears in Mario Super Sluggers and is one of the few characters on Wario's team in both games, along with Boo and Waluigi. He is once again a power hitter, and he also excels in pitching, with both stats being his highest, though his running and fielding are still rather poor. In Challenge Mode, he appears immediately after the player defeats Bowser Jr. at the Wario City stadium, stating that he turned invisible and was watching the whole game. However, he is uninterested in joining the player's team unless the team has some Boos with them. This is accomplished by both recruiting Boo and finding the Mini Boos item. King Boo has good chemistry with Petey Piranha, Boo, Wiggler, and King K. Rool, but he has bad chemistry with Mario, Luigi, Yoshi, Baby Mario, Baby Luigi, Baby Peach, and Baby Daisy.

His baseball bat no longer has a physical crown on it. In this game, it is represented as a crown picture above the "Boo mouth" picture. As well, in this game only, the colors of the jewels on his crown are reversed.

Super Princess Peach
King Boo aids Bowser in Super Princess Peach by appearing as the boss of Shriek Mansion. After adventuring through Vibe Island, Peach discovers where King Boo is and engages him in combat. During the battle, King Boo fires blue homing fireballs at Peach and chases her regardless of whether she looks at him or not. Peach can damage King Boo with direct light, which is made by having his own flames light the candles spread throughout the room. After a few attacks, King Boo uses his Gloom vibe, which causes him to cry, which extinguishes the candle flames. After enough attacks, King Boo is eventually defeated. With King Boo eliminated, Peach rescues the Toad and continues against Bowser's forces.

Mario Party 8
King Boo makes his first appearance in a Mario Party game in Mario Party 8. He gives out Stars on his own board King Boo's Haunted Hideaway. He hides in one out of three altars and gives the player a Star for ten coins if they can find him where he is hiding. If the player is able to buy the Star, he starts to "summon the bizarre" and disappears, causing the player to jump into a Warp Pipe that returns them to the starting point. This "makeover mojo" causes the whole layout of the course to change. However, if they do not have enough coins, King Boo causes a pitfall to appear under the player, causing them to return to the starting point and allowing the next player that reaches him to buy the Star. Additionally, if the player fails to steal anything from the board event Pickpocket Pinball, the hosting mouse sounds like King Boo. The player can purchase King Boo sounds from the Fun Bazaar, along with a figurine titled "King Boo's Revenge". When the player presses on the figurine during the Figurine Parade, King Boo floats up and gives off a brilliant glow, as if he is a light bulb.

Mario Party 9
King Boo appears as the stage boss of the board Boo's Horror Castle in Mario Party 9. He is fought in the minigame, King Boo's Puzzle Attack, where the competitors line up power-ups in lines of three or more to deal damage to him. Once his health reaches halfway, he throws Boos onto the field which can stun the player's cursors, making them unable to move until they shake the Boo off. What is unique about his boss battle is that if a player gets the Final Attack, they get ten points instead of the usual three, likely because the players can easily accumulate points up to fifty. In the final fight against Bowser, King Boo can also appear to hinder the players' progress. If Bowser throws the die with all the stage bosses' faces on it and King Boo's face appears on top, King Boo materializes above the stage. Then, he summons five Boos that move in a counterclockwise direction while moving outwards. When the Boos reach the boundaries of the stage, King Boo summons another five Boos, except this time they move clockwise. After that, King Boo disappears from the battle unless his face appears on the top of the die again.

Like most other characters in the game, King Boo has a constellation that can be returned to the sky; it is called the Ghost Major. It is located in the farthest left portion of the sky, near King Bob-omb's constellation and right next to Boo's constellation. Like all constellations, it can be bought for fifty Minigame Points. Also, in the Museum's Sound Test, there are two voice sounds with the name "King Boo's Entrance"; one is of King Boo and the other is actually of Boo.

Mario Party 10
King Boo reappears in the next title, Mario Party 10. He returns as a boss in the minigame King Boo's Tricky Tiles. In order to defeat him, the player must remember which platforms are false, and then run around until they can jump on a light switch (one is worth 3 points, and another worth 1 point). Like other bosses, King Boo gets angered after the players deplete his health halfway. The technique that King Boo uses is that he swaps Boos with platforms and he blocks the camera, so the player can't see the platforms very well and remember which ones are false. He can also be seen on the Luigi Board, which appears in the game's amiibo Party mode.

Mario Party: Star Rush
King Boo returns as a boss in Mario Party: Star Rush in his minigame, King Boo's Light Smite. In order to defeat him, players have to use Light Boxes which they need to carry over rotating platforms while evading Peepas. Once he loses half his health bar, the rotating platforms start moving faster and more Peepas appear, making it more difficult to reach the other side.

Mario & Sonic at the Olympic Winter Games
In the Wii version of Mario & Sonic at the Olympic Winter Games, King Boo appears as the first and sixth (in Individual Festival mode only) rival. King Boo races the player in the Alpine Skiing Downhill event in both Individual and Group Festival modes but he only races the player in Giant Slalom in the Individual Festival mode. King Boo has no skis so he goes down the hill smoothly. If the player loses to him, the guide (either Cream or Toad) says that it is unfair because King Boo has no skis.

In the Adventure Tours mode in the DS version, King Boo is the mid-boss of Polastraits. When the challenger approaches him, King Boo tells Mario/Sonic that he must race someone afraid of him. He selects Luigi and they must race in Giant Slalom. In this version, King Boo is transparent and the player can ski through him. He also appears slightly larger than in his other appearances.

Mario & Sonic at the London 2012 Olympic Games
King Boo appears in the next installment of the Mario & Sonic series, Mario and Sonic at the London 2012 Olympic Games. In the Wii version he is one of the characters that can be found in the London Party mode, and when he is encountered, he challenges the player to the Dream Sprint event. Since he has no feet, though, it is questionable how he gets the ball to roll. If the player reaches the goal line before him, they get a sticker as an award.

In the 3DS version, King Boo teams up with a Boo to face Sonic and Tails in the Badminton Doubles event. After winning, the player completes Sonic's second episode.

King Boo is one of the three characters who appear as rivals in Mario & Sonic at the London 2012 Olympic Games but do not appear as playable characters in Mario & Sonic at the Rio 2016 Olympic Games. The other two are Boo and Magikoopa.

Mario & Sonic at the Sochi 2014 Olympic Winter Games
King Boo appears in Mario & Sonic at the Sochi 2014 Olympic Winter Games as a rival in the Legends Showdown mode. He is found in Area 4, and is raced against in Short Track Speed Skating 1000m.

Mario Golf: World Tour
King Boo makes a cameo appearance in Mario Golf: World Tour on Hole 4 of Bowser's Castle. He is floating in the background alongside other Boos, and he is also much larger than in other games. His name can also appear on the tournament scoreboard.

Super Smash Bros. for Nintendo 3DS
King Boo appears as a trophy in Super Smash Bros. for Nintendo 3DS.

Super Smash Bros. Ultimate
He also appears in his Luigi's Mansion: Dark Moon design as an Advanced-class Neutral-type primary spirit in Super Smash Bros. Ultimate, being the enhanced form of the Boo spirit after it has reached level 99. When equipped, the spirit grants the fighter slower fall speed after jumping and has 3 support slots. King Boo is listed as the 49th spirit in the American English version, though he is listed as the 121st spirit alongside the rest of the Luigi's Mansion series spirits in the British English version of the game.

Puzzle & Dragons: Super Mario Bros. Edition
King Boo appears as a boss and enemy in Puzzle & Dragons: Super Mario Bros. Edition, mostly appearing at the Ghost House courses. His main attribute is Dark, while his sub-attribute is Light. As an ally, King Boo's skill can heal 1000 HP, and change all Heart Orbs into Dark Orbs. His Awoken Skill raises the amount of time that the player has to move Orbs around.

As an enemy, King Boo is capable of decreasing the player's maximum time to move Orbs around. Also, King Boo is treated as a generic enemy, so he can still appear as an enemy, regardless of whether or not the player has King Boo in their party. Big Boo can be transformed into a King Boo with six Boo Mushrooms as a Transformation Item, making it possible to create multiple King Boos on the player's team.

When King Boo attacks, a bunch of Boos attack with him.

King Boo's best stats are his ATK and RCV, while his HP is at an average level.

Mario & Luigi: Paper Jam


King Boo appears as a boss in Mario & Luigi: Paper Jam, making his official Mario & Luigi series debut. King Boo has a Papercraft version of himself controlled by Morton and Lemmy, which is fought in Bowser's Castle. The Mario Bros. use Papercraft Yoshi to fight it.

The real King Boo is encountered and fought in Gloomy Woods after the trio separate, leading to him kidnapping Mario and Paper Mario; leaving Luigi alone to find them, like in the Luigi's Mansion series. After finding Mario, he and Luigi begin to look for Paper Mario. They eventually find King Boo instead, disguised as Paper Mario. When they group together, King Boo reverts back to normal and laughs at the Bros. for a second, before re-transforming; Mario and Luigi then turn around, and are unaware of King Boo's presence. When they turn around again, King Boo reveals himself again, but is caught by the duo. In an attempt to fool them, he immediately reverts back to Paper Mario, and tries to convince the Bros. that he is the real Paper Mario, but fails. King Boo then loses control of his transformation powers, as he quickly transforms into Paper Peach and then Paper Bowser, before finally gaining control and giving up; reverting back to his true form, causing the battle to commence.

King Boo attacks by spitting blue fireballs at either Mario or Luigi before inhaling them; making them need to avoid the fireballs twice, and splitting into five copies before floating slowly towards the trio, turning invisible as he gets near; which the trio must counter using their hammers around 2 seconds after a copy turns invisible. During the battle, King Boo calls a Paper Boo holding the real Paper Mario onto the battlefield; threatening to harm Paper Mario if the bros attack either himself or the Paper Boo. In order to save Paper Mario, Mario and Luigi must defeat the minion holding onto Paper Mario, causing it to drop him. If King Boo is attacked while the Paper Boo is on the battlefield, he deals damage to Paper Mario. After a few turns, another Paper Boo can grab Paper Mario again, taking him out of the battle (which gets rid of Paper Mario's copies) temporarily. Finally, King Boo also has a chase attack where he grows bigger before attempting to shoot a giant blue fireball at the trio; which Mario and Luigi have to counter by throwing Paper Mario's boomerang form into King Boo's mouth while it is open. Once defeated, Paper Mario is reunited with the duo and the group continues to Mount Brrr.

Due to his crown, King Boo is immune to the brothers' Jump attack (but he can be hit by Hammer attacks, unlike Paper Boos) and certain Bros. Attacks, such as the Rocket Blast and Mega Thwonk's jumping attacks (only in the arcade). Attacking King Boo using attacks from above results in the Bro. taking 1 damage as if they were jumping on a spiked enemy. The Dropchopper, however, can circumvent this as it can be aimed towards King Boo's forehead instead of his crown and can then damage him.

The splitting attack used by King Boo is a reference to (if not a direct copy of) the Dark Star's splitting attack in Mario & Luigi: Bowser's Inside Story, except King Boo takes damage if countered unlike the Dark Star's version due to it being invincible. As well, if the Mario Bros. continue to attack King Boo without freeing Paper Mario, his slapping Paper Mario may be an allusion to Lady Bow and her slap attacks. Also, King Boo's voice clips from Mario Kart Wii are reused.

Mario + Rabbids Kingdom Battle
King Boo makes a cameo appearance in Mario + Rabbids Kingdom Battle in Spooky Trails, where an image of him can be found on the ground in the area where the gramophone is found.

Mario Tennis Aces
King Boo appears in Mario Tennis Aces as part of the Boo Hunt Co-op Challenge available from version 2.0.0. When Frenzy Time is active, King Boo appears and hovers around the court. Players can continuously hit tennis balls at him to earn more coins. He disappears once either his health bar or the Frenzy bar is depleted. In this appearance, he does not use any of his normal voice clips, instead using those of a regular Boo.

Physical appearance
King Boo's appearance has changed throughout his video game history. In his debut appearance in Luigi's Mansion, King Boo has dark red eyes, a blue tongue, two vampire-like fangs, a greenish complexion, and a ruby in a gold, round-edged crown. In Mario Kart: Double Dash!! and most games thereafter, he resembles a white Boo with a golden, five-pointed crown, though he is noticeably larger; additionally, his tongue is pink. As well, the crown has a blue jewel on the front and four red jewels around the sides in every game except Mario Super Sluggers, where the colors are reversed. In the later Luigi's Mansion games, he mostly resembles his original appearance, although he retains a full row of four teeth. He also has his eyes surrounded by a black shadow, a dark indigo mouth and tongue, and his crown has a purple jewel and pointed edges. He has also lost his original green tint, now ranging from purplish to pure white. In the Nintendo 3DS remake of Luigi's Mansion, he resembles his later Luigi's Mansion series appearances, but with his original crown. Like other Boos, King Boo is generally shown with an open, wide grin; however, when looked at, he bashfully hides his face with his hands. In certain games like Mario Party 9, King Boo's emotions are denoted by an aura surrounding him: when happy, he glows with a blue aura; when angry, he glows with a red aura. Sometimes he also has steam coming out of him to show that he is angry. Luigi's Mansion 3 shows that King Boo is more expressive since he, upon being first found by Luigi, scowls at him. His new speech icon has the original green shading, despite it not being present in his model, while his portraits show him with a purple crown.

Personality
King Boo is shown to have a deep hatred for the Mario Bros., especially Luigi after the latter defeats him in Luigi's Mansion. He says that he fears nothing except the Poltergust 3000, although in Super Mario 64 DS he admits that Wario's face scares him, rendering the previous statement a generalization. In the Mario and Sonic series, he has a superiority complex and is quite prideful. In Mario & Sonic at the Olympic Winter Games, he claims that he is unbeatable at the Alpine Skiing events, though this is proven false when the player beats him. In Mario & Sonic at the London 2012 Olympic Games, he claims to have beaten Mario before.

King Boo also loves to scare and toy with his opponents, rather than defeating them quickly. He chose to use his powers to blow Luigi back to the foyer instead of trying to finish him off while at maximum power, and enjoyed watching Mario struggle from within the painting he was trapped in, stating Mario's desperate attempts to break free were "satisfying."

Despite his huge size and immense power, he still covers his face in fear when looked at, much like a normal Boo, showing that he still has a shy nature like his minions; however, also like a normal Boo, he attacks when his foe's back is turned. King Boo has a distinct cackle, which is significantly lower in pitch than other Boos. He also has an echo in his roar.

King Boo actually considers his Boo minions as friends and cares for them. This is shown in Luigi's Mansion where he goes to free Boolossus after he is captured by Professor E. Gadd, and when he threatened to hurt Paper Mario if the Mario Brothers dared to hurt his Paper Boo minion in Mario & Luigi: Paper Jam. He also has good chemistry with the playable Boo in the Mario Baseball series. In Mario Super Sluggers, King Boo says that he finds little Boos cute. He does not seem to care for ghosts that are not Boos, instead using them in his plans as tools rather than subjects. Even Hellen Gravely, a ghost who adored him, was seen as nothing more than a pawn.

After breaking out of the portrait he was placed in at the end of Luigi's Mansion, he displays a complete disregard for anything that interferes with his revenge and displays several sociopathic traits: including a disregard for others' well being, and at one point nearly causes the reality of the Mushroom World to collapse. Professor E. Gadd comments on how King Boo redecorated the Treacherous Mansion to make it scarier. He also goes into fits of rage when his plans do not go the way he wants. At least in Luigi's Mansion: Dark Moon, King Boo also has a bit of a sarcastic demeanor as well, as when formally revealing himself to Luigi before fighting him, when referencing his earlier defeat at the hands of Luigi, he says "Good times" over the recollection of the event despite obviously being angry and vengeful over the earlier defeat. In Luigi's Mansion 3, King Boo impersonates Mario and Luigi's Italian accent saying, "It's-a me, King Boo!" and uses false comfort towards Luigi. Also in said game, in addition to his increasingly more vengeful and anti-social nature, he is also shown to have a degree of cunning, as it was strongly implied that he deliberately had Hellen Gravely guard only Mario's portrait specifically because he anticipated that Luigi would ultimately defeat her, and used Princess Peach's portrait as bait to draw out Luigi alongside Mario in order to recapture all the portraits, with him very narrowly failing in this endeavor.

Powers and abilities
Besides possessing the abilities of other Boos, including the ability to teleport and turn invisible, King Boo has a numerous amount of other powers. Notably in Luigi's Mansion, King Boo's power increases when he is in the presence of other Boos. For example, King Boo has the power to blow Luigi to the Foyer when enough Boos are in the mansion. King Boo also has the power to materialize things, including pulling a highly detailed mansion out of the ghost dimension and creating lifelike paranormal replicas of some of Mario's enemies. He has shown the ability to create Boos or dimensions to battle in and even bubbles and blue fire, which he spits out of his mouth. King Boo is also shown to be able to regenerate from his crown in Super Princess Peach. He is shown to also have the power to grow in size as seen in Luigi's Mansion: Dark Moon.

In Luigi's Mansion: Dark Moon, King Boo exhibits new abilities such as being able to open paranormal portals (which lead to the ghost dimension), create traps such as sand patches, electrify the terrain to shock Luigi, and teach Boos how to create Spirit Balls. His new crown can also shoot an ectoplasmic laser that can shatter the Dark Moon or trap Toads within paintings. King Boo can create illusions, hence his title "The Master of Illusions." His power is also no longer reliant on the presence of other Boos, although it is stronger when there are other Boos in the vicinity.

In Luigi's Mansion 3, King Boo is shown to summon magical paintings to trap people and to a certain extent objects inside. He can also enlarge the portraits in order to ensure he captures all his targets, as an extension of his paranormal portals. He also doesn't seem to even need the presence of other Boos to grow stronger. In addition, it's also implied that he can still maintain control of the ghosts via his crown gem even when he himself is incapacitated, as the ending had the ghosts Luigi captured swarming to attack Luigi, only to be restored to their senses after the gem vanishes from Luigi's hands.

King Boo appears with a rather high power stat in many of his spin-off appearances. This is most noticeable in the Mario Baseball series, where he is able to hit home runs with relative ease. His great strength is also shown in Luigi's Mansion: Dark Moon as he summons spike balls and makes them fall down, even the larger ones, simply by slamming the platform. King Boo also appears to be a good Alpine Skier, as he states that he is exceptional at that event, though this remains questionable due to his lack of skis (or legs for that matter).

In Mario & Luigi: Paper Jam, King Boo shows the ability to shapeshift, shown when he tries to trick Mario and Luigi by posing as Paper Mario, and later briefly transforming into Paper Peach and Paper Bowser. He also has the ability to spit blue fireballs.

Leadership
Although he commands a large band of Boos, not all Boos call King Boo their leader, with some Boos working directly for Bowser, and many others living peacefully with the other denizens of the Mushroom Kingdom, either independently or as part of a small band of Boos led by Lady Bow. Notably, those Boos that do claim allegiance to King Boo provide him with further power. King Boo has also been affiliated with other ghost enemies, like Peepas.

Enemies
King Boo initially considered Mario as his main adversary, however after his defeat at Luigi's hands during Luigi's Mansion, Luigi has become King Boo's arch-nemesis. Luigi and King Boo's relationship is often drawn in parallels with Mario and Bowser's, and to lesser extents, Yoshi and Kamek's, and Donkey Kong and King K. Rool's. King Boo also hates Professor E. Gadd for his ghost-capturing hobby, even stating that the only thing that scared him was E. Gadd's invention, the Poltergust 3000. In Luigi's Mansion: Dark Moon, King Boo reveals a desire to get revenge on E. Gadd as well as Luigi, by trapping the scientist within a portrait of his own something he would later succeed in doing in Luigi's Mansion 3. In Mario Superstar Baseball, King Boo has negative chemistry with Luigi and Baby Luigi; in Mario Super Sluggers, he is shown to also have bad chemistry with Mario and his infant counterpart, and even Yoshi and both princesses' infant counterparts. In Luigi's Mansion 3, Princess Peach is among King Boo's victims to be turned into portraits. Once Luigi rescues all the Toads, King Boo takes possession of Peach's picture, knowing that Luigi will come for her once he saves Mario from Hellen Gravely.

Friends
King Boo has been shown to work with Bowser on numerous occasions, including Super Mario 64 DS, Mario Party 9 and Super Princess Peach. He also used a Bowser decoy and the two share a portrait after King Boo's defeat in Luigi's Mansion. Despite their apparent alliance, the two share neutral chemistry in Mario Superstar Baseball and Mario Super Sluggers, with King Boo appearing on Wario's team, rather than Bowser's. King Boo has also been paired up with Petey Piranha as in Mario Kart: Double Dash!!, with the two sharing chemistry in the Mario Baseball games, although their relationship has never been fully explored. King Boo has good chemistry with Wiggler and King K. Rool in Mario Super Sluggers, though his relationships with both characters, like Petey Piranha, remain unexplored. Luigi's Mansion 3 shows King Boo having a dedicated fan in Hellen Gravely, who releases him to impress and aid him in his plot for revenge against Luigi. Despite her infatuation for him, King Boo sees her as just a pawn in his plan for vengeance. When Luigi frees nearly all his friends, leaving only Mario and Peach to saved, King Boo retains low faith in her capabilities and leaves her to be captured as part of his contingency plan; he eventually deems her useless for being unable to subdue Luigi and keep his friends imprisoned.

Luigi's Mansion series

 * ""What's that he's got on his back!? It's E. Gadd's Poltergust 3000! Everybody scram!" - Luigi's Mansion
 * "White gloves! A cute little moustache! Soft shoes! How did you get out of my...? ... Huh? Wait just a second... Your clothes are...green... Hey, you must be Luigi! Whoa, you had me a bit confused for a moment there! What? You want me to release Mario? Riiiiight. No, no, no. I will not give up my favorite decoration. I like Mario just where he is. Hey, so what's that you got there? A Poltergust 3000?! Uh oh! What should we do, Boos? Good idea, guys! All Boos in this area, gather your strength! We'll blow him away!!! ... We'll take a big Boo breath... ...and blow you away!!" - Luigi's Mansion
 * "Aahh, I could just stare at my Mario painting for hours! It's true what they say about fine art...it takes utterly refined sensibilities to truly appreciate it! The way you plead for help, Mario... I find it so...satisfying. Perhaps that is merely because I remember how much trouble you've caused me in the past. ... Seriously, though, who would actually believe that mansions get given away in contests?! Talk about stupid! What do they feed you Mario brothers anyway...gullible soup? Your brother came all this way just to get turned into a painting... It's just terrific. However... This Mario painting looks lonely... I must have a Luigi painting as well! Then my gallery will truly be complete! Bleah HA HA! I am not afraid of you, fool! I fear only that infernal Poltergust 3000 you carry on your back! Stupid machine! But I am a KING among Boos! I swear it... I shall fear no mere house-cleaning device!! I must say...I despise the way you SHWEEERRPP-SHLOOOOP-WHOOORPP-SHLEEEOOORG-vacuumed up all my friends! Don't imagine that I'll flee... I'll fight you like a true Boo! Now join your brother... ...inside the painting!" - Luigi's Mansion
 * "Mwah ha ha! Now what do we have here? Why, if it isn't my old pal Luigi! Or is it Baby Luigi? I can't tell the difference. Hey, remember that time you trapped me in a painting for all eternity? Good times. But guess what? I GOT OUT! And now I'm painting the town RED! Yes, it was I who broke the Dark Moon! This power-enhancing jeweled crown of mine made it easy. And now that all the ghosts of Evershade Valley are under my control, it's time to take my rightful place as supreme ruler of your world! You, that annoying scientist, and all your friends will look great hanging from the wall of my throne room!" - Luigi's Mansion: Dark Moon
 * "Gwahaha! If it isn't my good friend, Luigi! It's me, your ol' pal KING BOO! Y-You're not telling me you FORGOT ME!? Well, I certainly haven't forgotten YOU! You, that ratty vacuum cleaner, and that stinky old geezer who made it! I haven't forgiven what you did to me, so I've led the ghosts here in an uprising! But you... you and that DISGUSTING machine just SHLEEEERP-SHWOOOOOP-SWHLOOORPSHLOOOORP-SHWEEEERPed them all up! You think I'll let you get away with that!? You're going down this time, Green 'Stache!" - Luigi Mansion Arcade
 * "Mwah ha ha! Surprise, Luigi! It's-a me, King Boo! That old coot had me locked up tight in his lab... But guess what? I GOT OUT! The hotel owner was here just dying to meet me, so she busted me out. Pretty lucky, huh? Anyway, I didn't think that you'd actually show up here. I was ready though, just in case. In fact, my vengeance is nearly complete! What do I mean by "vengeance"? Glad you asked! I'm trapping you and everyone in your vacation party... in frames! Awww, there there, Luigi. It'll be over soon. It's time for a family reunion! BOO-YA! [...] Mwah ha ha! Take THAT! Oh, and there's one more frame, of course. I saved it especially for you! Stay right there and put on your best terrified face, Luigi! This is game over!" - Luigi's Mansion 3
 * "Mwa ha ha! how brash of you to come and face me, Luigi! You should have just run away while you had the chance! But instead you stayed and fought all the way up here to oppose me? How foolish! You may have made your way through the hotel using E. Gadd's frustrating inventions to suck up all the ghosts in your path... You were all SHWEERRPP-SHLOOOOP-WHOOORPP-SHLEEOOORG! Ahem. Anyway, just because you vacuumed up all of those ghosts doesn't mean you can take me on! It doesn't matter that you beat the hotel owner... Hellen Graveley! Ooooooo! She let all your friends get away! It's so frustrating when you trap something in a painting and it gets out, you know? Arg! Hellen Graveley... All those ghosts... Useless! I have to do everything myself! I'm sick of this! No more mercy for you, Luigi! No more tricks--no more carefully laid plans! Now it's time for raw King Boo gusto! You're all going in a painting! An ensemble portrait is your fate!" - Luigi's Mansion 3
 * "Mwa ha ha! Serves you right! Just a few left. Go on! Join them! My painting must be complete! Sorry to keep you waiting. Are you ready? I don't care!" - Luigi's Mansion 3
 * "...Hmm? One-two-three-four-five-six-sev-- WAIT! There should be seven! Who's missing!? [...] Luigi! What are you doing over there?! UGH! That's it! I'm sick to double death of you! You want to fight me? FINE! Let's go, Luigi! This is the end for you, once annd for all! HERE I COME!" - Luigi's Mansion 3

Super Mario 64 DS

 * (signpost) "Big Boo's Haunt. Eee hee hee...There are no exits. You may as well wander in here forever...Listen carefully. Can you hear me? If you find me, you're history. Eee hee hee..." - Super Mario 64 DS
 * (to Mario before getting Luigi's key) "Eee hee hee! I am Big Boo, the spookiest of all the spirits! I locked up the whimpy green moustache man. And I'll do the same to you! Eee hee hee! Since I'm a ghost, I can turn invisible in the mirror's reflection. You'll have to attack me from behind to defeat me. But you'll die of fright before that ever happens! And then you'll be one of us!" - Super Mario 64 DS
 * ''(to Mario after getting Luigi's key) "Eee hee hee... I am Big Boo, the spookiest of all the spirits. I love scaring wimpy humans. Eee hee hee... You'll never sneak behind me!"
 * ''(defeated) "Eee hee hee! You can never really kill a ghost! We always come back! See you again. Eee hee hee!" - Super Mario 64 DS
 * (to Luigi) "Eee hee hee! I am Big Boo, the ghastliest of all ghosts! What the...? I thought I locked you up...Well, never mind. Now I'll really creep you out! You can't escape...Eee hee hee!" - Super Mario 64 DS
 * (defeated) "Eee hee hee! We ghosts always rise from the dead. I just love the look of terror on your face! In fact, I live for it! Eee hee hee! See you soon!" - Super Mario 64 DS
 * (to Wario) "Eee hee hee! I am Big Boo, the scariest of the spirits. Eek! What's this?! Your face... It would even scare a ghost. How horrible! You've got it backwards... I do the scaring around here! All right, now I'll put some real fear in that frightening face of yours! Eee hee hee!" - Super Mario 64 DS''
 * (defeated) "Augh! The scariest one wins, and your face is flesh-creepingly frightening. Good thing I don't have any flesh! Eee hee hee! This is not the end. See you again." - Super Mario 64 DS

Mario & Luigi: Paper Jam

 * " Look at this little scrap. Friend of yours, is he? Well, if you like your friend, you better stop attacking me! Now, don't go thinking it's OK to attack my little minion either. Who knows what could happen if he dropped your little friend there... You got that?" - Mario & Luigi: Paper Jam

Mario Kart: Double Dash!!
Website Bio: ''If you're not afraid of ghosts, you will be once King Boo is through with you. To unlock him, finish the Mirror Mode Star Cup with a 1st place win.''

Luigi's Mansion: Dark Moon

 * Website Bios:
 * Flag of USA.png The orchestrator of the Dark Moon debacle is as regal as he is round...Can you bring peace back to Evershade Valley?
 * Flag of Europe.png This spooky sovereign shattered the Dark Moon, causing the ghosts to turn mischievous. Piece the Dark Moon back together and restore peace to Evershade Valley!

Trivia

 * When King Boo says, "I will not give up my favorite decoration. I like Mario just where he is," he references a quote from Star Wars VI: Return of the Jedi, when Jabba the Hutt says, "I will not give up my favorite decoration. I like Captain Solo where he is."