Red Shell

The Red Shell is a throwing item from the Super Mario series which also appears in Super Smash Flash 2. It is one of the 4 items from the shell family, which also includes the Green Shell, the Spiny Shell, and the Yellow Shell. Unlike the Green Shell, the Red Shell will automatically move towards the closest player (including the user), yet will not fall off of edges.

Origin
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 the Super Smash Flash series originates from its behavior in the Mario Kart series, where it homes in on other characters and causes the kart to flip out upon contact. Red Shells can backfire if the user comes in contact with it again after firing, such as in Mario Kart 64, where the red shells would eventually stop homing and start traveling along the course's path, hitting whoever was unfortunate enough to sit in front of its path. This was translated into SSF2 as well, though it is much easier for the user to hit themselves with it. Its behavior also references the behavior of Red Koopas, as they don't walk off of ledges, turning around instead.

In Super Smash Flash
Though Red Shells as items are missing in the original Super Smash Flash, there are Red Koopa Troopas found in the first Adventure mode stage, Super Mario World. However, this is only cosmetic because the Koopas instantly die in one hit without dropping their shells, thus preventing their use as items.

In Super Smash Flash 2
The Red Shell debuts in the Super Smash Flash series with the fan game Super Smash Flash 2, as it was present in the original Super Smash Bros. and Super Smash Bros. Melee (but ironically not in Super Smash Bros. Brawl, outside of the Subspace Emissary).

When the Red Shell is used, it slides across the floor and homes in on the nearest character. Though it doesn't deal as much damage as the Green Shell, it cannot be stopped once thrown, and will continue to move around and hitting enemies until it eventually disappears. It can be reflected, but this doesn't change its behavior. A player may escape from the red shell by not standing on the red shell's platform, since the red shell is incapable of flying or dropping off of a platform's edge while active.