Metal Mario

Metal Mario is a powered-up version of Mario debuting in Super Mario 64. Upon wearing the Metal Cap, the heroic plumber will be coated entirely in metal. In this form, he is impervious to any type of damage and is able to walk underwater due to how heavy he is.

Since the power-up's debut, Metal Mario has reappeared as a seperate entity from Mario. First appearing in the Super Smash Bros. series starting with the original on the Nintendo 64, he acts as a silent mid-boss and is characterised like a statue. With his resurgence in Mario Kart 7, he's given a filtered voice done by Charles Martinet saying spins on Mario's own catchphrases.

Theorising
Despite never butting heads with the character he takes his form from, Metal Mario is referred to as Mario's rival in official descriptions of the character. It's strange how Metal Mario has become his own character despite never appearing as such in his original appearance, and it has led many to believe that he was originally intended to have a bigger role in Super Mario 64.

Trivia

 * The texture Metal Mario uses is the default metal texture of the SGI workstation. Oddly, Super Mario 64 is the only game this texture is used; in his future appearances, such as Super Smash Bros., he uses a different texture. Even with Metal Mario's return in Super Mario Odyssey using the game model from Super Mario 64, he still uses a different metallic texture. This could relate to the Silicon Graphics curse on the iceberg.
 * The metal texture is also used in The Legend of Zelda: Ocarina of Time.
 * The same metal texture used in Super Mario 64 is set to return in Super Mario 3D All-Stars, marking the texture's first reappearance since the original Super Mario 64.