The Red Shell (アカのこうら, Red Shell) is a throwing item in Super Smash Bros. and Super Smash Bros. Melee that will actively hunt out the nearest player, even the player that activated it. Red Shells appear as special items in later titles without the homing behavior, acting mostly the same as Green Shells, which appear as normal items in those games.
Red Shells have appeared in every Mario game that has a Red Koopa Troopa ever since Super Mario Bros.; however, the way the Red Shell acts in Super Smash Bros. and Melee originates from its behavior in the Mario Kart series, where it homes in on other characters causing the kart that got hit to flip out in an attempt for the firer to steal a place. Also, the red shells can backfire too just as it can in Smash Bros. Its behavior in the Smash Bros. games could also be referencing the behavior of Red Koopas, as they do not walk off of ledges like other enemies do.
Although based on the Red Shell from Mario Kart, Red Shells also act similarly to the Kamikaze Koopa from Super Mario World. When a yellow Beach Koopa entered any shell, they would begin to pursue Mario or Luigi the best they could, while still sliding on the ground.
Instead of sliding in the same direction it was thrown in after coming in contact with the ground like a green shell, it will start sliding from left to right, targeting the closest player. If there are no fighters on the same platform as a Red Shell, it will continuously slide from side to side; much in the same way as Red Koopa Troopas turn around at the end of a platform, a Red Shell will not fall off the platform on which it is moving, and instead will bounce back from the edge. When stationary, the Red Shell will activate when jumped on or after coming in contact with a hitbox. Like the Green Shell, a sliding Red Shell will halt all movement upon being jumped on. The Red Shell does moderate set knockback and therefore cannot KO opponents at any percentage except lower handicapped opponents. When the Red Shell is reflected, its damage output is increased, but its knockback is not. A reflected Red Shell will also have much more hitlag when it hits a player.
The Red Shell does not appear in Super Smash Bros. Brawl, but Red Koopa Shells appear in the Subspace Emissary. However, these function in the same way as a Green Shell. The same goes for Smash Run in Super Smash Bros. for Nintendo 3DS, as the Red Koopa appears once again as an enemy.
The Red Shell also appears as headgear for Mii Fighters in SSB4.
Red Koopa Shell
The Red Koopa Shell is a special item that appears in the Adventure Mode of Melee, The Subspace Emissary of Brawl, and Smash Run of Super Smash Bros. for Nintendo 3DS, which can only be obtained by attacking a Red Koopa Paratroopa or Red Koopa Troopa. In Melee, only one Red Koopa Paratroopa appears. Although similar (but not identical) in appearance to a Red Shell, it behaves like a Green Shell, and does fire damage in Melee, a reference to when Yoshi can eat red shells and spit fire as a result in Super Mario World. When compared to the Green Koopa Shell, it does even more damage and knockback. Another difference from the Red Shell is if it is left alone for a while, it will turn back into a Red Koopa Troopa.
Throw a Red Shell, and it will home in on random characters and send them flying straight up. Red Shells moving around the surface are dangerous to all players, but they may also present attack chances for players with good timing and strategy. Red Shells won't leave the platform they're on once they start moving.