Shell Mario

Shell Mario (also known as Koopa Mario ) is a form of Mario that Mario can become in New Super Mario Bros. by touching a Blue Shell. It allows him to use Frog Suit-like control while swimming, and gives him the Hammer Suit-like ability to dodge the attacks of enemies when he crouches, tucking into it. If he ducks while running or runs downhill or on level ground for long enough he will hide in the shell and the shell will slide through the world. This maneuver is called a Shell Dash. As long as he keeps ducking, he'll be safely protected from most enemies. However, it won't protect him 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 back in Mario & Luigi: Bowser's Inside Story. The Blue Shell is an item found in the overworld. If Mario and Luigi are outside Bowser's belly, 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 cant directly go into. 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.

Trivia

 * In any of the boss fights with Bowser Jr, Mario or Luigi can use the shell dash to knock the young koopa out, similar to what the player do in later fights, where the player needs to throw koopa shells at him before jumping on him.