Red Shell

"Throw a red shell, and it will zero in on the enemy!"

- Penguin

Red Shells are red Koopa Troopa shells that first appeared in Super Mario Bros. Although not their first specific use, they were worn by Koopa Troopas. Unlike the green-shelled Koopa Troopas, they did not fall off cliffs and simply turned around when reaching a ledge. They are the second most common shell, after Green Shells.

Super Mario Bros.
The Red Shell's first appearance was in Super Mario Bros. They are found being worn by Koopa Troopas. Red-shelled Koopa Troopas appeared later than green-shelled Koopa Troopas. Despite their different color, they act in the same manner as Green Shells. Once tossed, it will continue until jumped on or dropped down a cliff. This may prove as a valuable projectile, defeating most enemies, but may damage the player when it is touched while moving.

Super Mario Bros.: The Lost Levels
Super Mario Bros.: The Lost Levels returned Red Shells along with Green Shells. Little or nothing was changed about the behavior of Red Shells in this game from Super Mario Bros.

Super Mario Bros. 2
In Super Mario Bros. 2 (which calls them "Turtle Shells"), they are the only type of shells and are found in grass rather than on Koopa Troopas. The player can throw it at enemies just like the vegetables in the game. It will continue sliding until it hits a wall, then it will disintegrate rather than bounce back. It is possible, with some difficulty, to actually ride the shells by getting on top of it. They replaced the Big Face from Yume Kōjō: Doki Doki Panic.

Super Mario Bros. 3
Red Shells returned in Super Mario Bros. 3. They acted in the exact same manner as in Super Mario Bros., except they could now be carried and thrown unlike Super Mario Bros., where they can only be kicked.

Super Mario World
During the events of Super Mario World, Yoshis can spit out fire while in possession of the Red Shell. Green and Red Yoshis will spit out fire upon eating one. Also, Red Shells can only be obtained by stomping a Koopa Troopa out of its shell, making it become a Beach Koopa. If a Beach Koopa returns to its shell, it once again gains possession of the red shell.

New Super Mario Bros.
Red Shells appear once again in New Super Mario Bros. Again, they are only obtained by defeating Red Koopa Troopas who hold the shell. No change has occurred between New Super Mario Bros. and in Super Mario Bros. regarding functions.

Super Mario Galaxy
Red Shells reappear in Super Mario Galaxy as an offensive weapon. Like the Green Shells in the game, Mario can only hold the Red Shell, not ride it. If Mario throws the Red Shells near an enemy, it will home in that enemy until it hits something. Underwater, it allows Mario to swim faster and with greater ease. It can also be thrown at underwater creatures and home in on them as well. The other variety of shell that appear in Super Mario Galaxy are Gold Shells, which act like Red Shells when thrown.

New Super Mario Bros. Wii
Red Shells reappear in New Super Mario Bros. Wii as a projectile. Little or no change has occurred between the game and its predecessor regarding functions.

New Super Mario Bros. 2
Red Shells appear again in New Super Mario Bros. 2, once again as a throwable weapon. Nothing major has changed, though if the player runs through a Gold Ring and there is a Red Koopa Troopa in the area, they will turn Golden, as does their shell. Once thrown, the shell produces a trail of coins behind it.

New Super Mario Bros. U
Red Shells appear again in New Super Mario Bros. U, where they act in the same way as in the previous games, Giant Red Shells also appear.

Super Mario 3D World
Red Shells appear in Super Mario 3D World not as a item, but as a color swap for the Green Shell when Mario is inside it. Other colors include Pink Shells and Blue Shells. They also appear in the second Boom Boom battle, here Boom Boom uses them to attack Mario while he uses his spin attack.

Mario Kart series
In the Mario Kart series, Red Shells have the ability to home in on the opponent whose rank is one higher than the user's. Red shells can be picked up in sets of one or three. When the triple variant is used, three Red Shells rotate around the driver's kart upon usage, (all games except Mario Kart: Double Dash!!) protecting them from Bananas and other shells. If the driver collides with another driver using Triple Red Shells, they will be hit by one, and if the other driver also has another set of Triple Green Shells or Triple Red Shells, both drivers lose their shells. Triple Red Shells are not used in Mario Kart DS Wi-Fi mode because items cannot be equipped in that mode. In nearly every game of the Mario Kart series, Triple Red Shells are a player exclusive item. The only exception is in Mario Kart 7, where, if the item settings are shells only, the AI is able to use Triple Red Shells.

In Mario Kart: Super Circuit, single Red Shells can be laid behind the player and act as a land mine, chasing after the first kart that passes by, even the user.

In Mario Kart: Double Dash!!, the Triple Red Shells are a special item exclusive to Koopa Troopa and Paratroopa (and Petey Piranha and King Boo, due to their ability to have any special item). Also, in Double Dash!!, the three shells are carried in their hands rather than having them circling around the kart. When the kart is hit by an item, the three shells are reduced to one.

In Mario Kart 7 onwards, Red Shells are able to trail in the sky, unlike older Mario Kart games, where Red Shells fall when thrown in the air, just like a Green Shell when it is thrown in the air.

In Mario Kart 8, they follow the same rules as in past installments and can also come in single or triple.

Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
Unlike the main stream Mario games, Koopa Troopas appear in rarer cases in Super Mario World 2: Yoshi's Island. A Red Koopa Shell could be used by spitting it out, but it cannot be used for making eggs.

Yoshi's Island DS
Just like its preceding game, Koopa Troopas wearing the red shell are a rare find in Yoshi's Island DS. They are used in the exact same manner as in Super Mario World 2: Yoshi's Island.

Super Smash Bros. series
Red Shells also appeared in the Super Smash Bros. series. They were in both Super Smash Bros. and Super Smash Bros. Melee. Unlike the Green Shell, which went in a straight path until it fell off, the Red Shell will stay on one platform, and bounce back and forth hitting anyone who hits it (including the user). They disappear over time, however. Notably, the Red Shell is absent from Super Smash Bros. Brawl.

Paper Mario: Sticker Star
Red Shells appear as the shiny variant of Green Shells. Also, every Paratroopa in the game has a Red Shell on its back.

Mario & Luigi: Partners in Time
In Mario & Luigi: Partners in Time, the Red Shell is a Bros. Item that is used as a weapon against enemies. When the item is chosen, one of the brothers will kick the Shell towards the foe. It will then bounce off the enemy, cause damage, and come to the other player. After each kick, the shell starts moving faster along the ground. Unlike with the Green Shell, which is only capable of dealing 16 hits, the brothers can then repeat this indefinitely until the player misses a kick. Also unlike the Green Shell, the Red Shell will move on and target another enemy if the chosen enemy is defeated. When the adult and baby Mario brothers fight together, the babies can sit over the shell and spin upon hitting the enemy, causing extra damage to the foe.

Mario & Luigi: Dream Team
The Red Shell reappears as the first of Mario's Bros. Attacks in Mario & Luigi: Dream Team, under the name 3D Red Shell and it costs 4 BP to be used. The Attack Pieces to obtain the 3D Red Shell are found in Mushrise Park, which are required to all be collected to proceed through the game. It can only target enemies on the ground. The 3D Red Shell is more focused on enemies than its counterpart used by Luigi.

Upon using the attack, Mario and Luigi will run into the foreground, where Mario will take out a Red Shell and kick it at one of the enemies. Luigi will kick the Red Shell back at the enemies after it rebounds off the enemy towards him, and Mario will do the same, this repeating three more times. After the Red Shell hits nine times, Luigi will then kick the Red Shell towards Mario, who blasts it at one of the enemies, ending the attack. If all enemies are defeated before the attack ends, it will end prematurely.

Striking three, five, nine and ten times with the Red Shell will yield an "OK!", "Good!", "Great!" and "Excellent!" rating respectively.

Description
Use the A Button for Mario and the B Button for Luigi, and kick that shell!

Mario & Luigi: Paper Jam
The 3D Red Shell returns in Mario & Luigi: Paper Jam, functioning the same as it did in Mario & Luigi: Dream Team. However, the move is now available from the beginning of the game, costs 3 BP instead of 4, and the attack now damages enemies considerably more, at the cost of being shortened to six hits rather than ten hits, also the speed of moving forward and the shell are significant increased.

Striking one, three, five and six times with the Red Shell will yield an "OK!", "Good!", "Great!" and "Excellent!" rating respectively.

Mario Strikers series
In the Mario Strikers series, the item works very similarly to the Green Shells, which are also found in the games, however, they differ slightly in appearance and effect. They are red, and instead of moving in one direction, they home in on the opposing players. Every time this item is collected, normally is obtained in a group of three. This allows it to home in on three opposing players, however, one will not get hit. These can be used when trying to perform a Super Strike, a Mega Strike, or a Skillshot. There is also a very small chance that the shells might miss the opposing team members, and start bouncing around the stage like a Green Shell. The giant variety of this shell also appeared in the games. It can be useful for clearing room for a Super Strike or Mega Strike.

Mario Sports Mix
Red Shells also appear in Mario Sports Mix. When used normally, they'll chase after an opponent, and temporarily knock out anyone in their path. When combined with a ball/puck, the object will do a curved path at high speeds. The curve can occasionally throw off opposing players.

Mario Kart 7
"Homes in on the closest kart in front of you and knocks it over."

- Physical Game Manual