Shell Mario

Shell Mario (also known as Koopa Mario ) is a form of Mario and Luigi that they can become by touching a Blue Shell.

New Super Mario Bros.
Despite Shell Mario making its debut in New Super Mario Bros., it allows them to swim faster, and gives them the Hammer Suit-like ability to dodge the attacks of enemies when they crouch, tucking into it. If they start running for a short time or crouches while on a downward hill they will hide in the shell and the shell will slide through the world. This maneuver is called a Shell Dash. As long as they keep ducking, they'll be safely protected from most enemies. However, it won't protect them from Bob-omb explosions, spike balls, hammers thrown by Hammer Bros. or Sledge Bros., boomerangs from the Boomerang Bros., and the Raining Debris from World 8-8. Shell Mario is extremely powerful in this form as he is able to defeat enemies such as Thwomps and Boos while doing the Shell Dash. However, the shell power-up also makes it almost impossible for Mario or Luigi to Triple Jump, while Double Jumping becomes harder to perform because the player must make the second jump while running but before Mario goes into the shell to dash.

Mario & Luigi: Bowser's Inside Story
Shell Mario returns in Mario & Luigi: Bowser's Inside Story. The Blue Shell is an item found in the overworld. Mario can go in Blue Shells by striking Blue Shell Blocks which were spread around when Mario and Luigi hit a giant version of it, and turn into Shell Mario. Luigi can then hit Mario using his hammer, and this will make Mario go into the shell, spinning straight forward. This can be used to hit blocks in areas which Mario and Luigi can't directly go into. However, because the shell is so heavy, Mario is unable to jump in it. If he does, he loses the shell. It serves a similar purpose to the Kooper and Koops partners in the Paper Mario series. This power-up was introduced by a group of Blue Paratroopas.

Super Mario 3D World
While the form itself doesn't appear in Super Mario 3D World, if a player dashes while carrying a shell, they will be able to duck into the shell and dash similar to Shell Mario.

Trivia

 * Shell Mario is the only power-up from New Super Mario Bros. that isn't seen in New Super Mario Bros. 2.