Red Shell

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.

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. Any type of Yoshi will spit out fire if they eat a red shell. 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.

Super Mario World 2: Yoshi's Island
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.

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.

New Super Mario Bros. Wii
Red Shells reappear in New Super Mario Bros. Wii as a projectile. Little or no change has occurred between New Super Mario Bros. Wii and New 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.

Mario Kart Series
In the Mario Kart series, they can home in on the opponent whose rank is one higher than the driver's. Red shells can be picked up in singles or triples, in which case they circle around the kart until all three are fired. In this state, they can be used as a barrier, useful for blocking contact with other hazards, such as Bananas. However, they offer no protection against Blue Shells and Fake Item Boxes. If the character comes in contact with any other driver, then the other driver will be knocked over. The exception to this rule is in Mario Kart: Double Dash!!, where the shells are carried in the hand rather than circle around a character's kart. In Mario Kart Double Dash Mario Kart DS Wi-Fi mode, a shell could not be dragged by holding the item button.

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.

Mario and Luigi Series
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 15 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 toddler Mario brothers fight together, the babies can sit over the shell and spin upon hitting the enemy, causing extra damage to the foe.

Mario Football Series
In the Mario Football soccer game 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. Everytime 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.