Metal Mario (character)

Metal Mario is a powered up form of Mario, which first appeared in Super Mario 64. Coating Mario in a metallic sheen, Metal Mario is tougher, stronger, virtually invincible, and can only be hurt by long falls. This form is acquired by picking up the Metal Cap from green ! Blocks or Metal Boxes. Metal Mario has appeared later as an independent character in the Mario universe, usually being one of the fiercer endgame opponents in the Mario sport games or Super Smash Bros. series, boasting high power and endurance stats.

Other characters can actually turn into metal as well.

Super Mario 64 and Super Mario 64 DS
When Mario uses a Metal Cap in Super Mario 64, he becomes Metal Mario, a living embodiment of metal. In this form, Mario is invincible and can destroy enemies just by running into them. Metal Mario is also very heavy, and as such, can withstand strong winds, underwater currents, and can walk on the bottom of bodies of water. In addition, he doesn't need to breathe, meaning he can stay underwater without worrying about air until the metal power runs out. However, Metal Mario is slower than the regular Mario and sinks in quicksand faster.

In the Super Mario 64 DS version of the game, only Wario could turn metallic and become Metal Wario by using a Power Flower.

Mario Golf 64
Metal Mario is an unlockable playable character in Mario Golf 64.

Dr. Mario 64
He is also the final boss of Wario's storyline in Dr. Mario 64. In this game, Metal Mario can be unlocked as a playable character by beating the entire game without losing once.

Super Smash Bros.
In Super Smash Bros., Metal Mario is a stubborn mid-boss before the game's final boss. He is far less powerful and is susceptible to much more damage than in Super Mario 64. Nevertheless, he is a much more durable character than the regular Mario. Heavier and more resistant to knockback in general than any other character in the game, he's very difficult to beat. In fact, only when Metal Mario's damage exceeds 180% can he be knocked down.

Super Smash Bros. Melee
He returned as a mid-boss in Super Smash Bros. Melee and as one of the game's many trophies. In this game, Metal Mario would occasionally fight alongside his brother, Metal Luigi. Additionally, players could pick up the Metal Box item and gain metal forms for a limited time, boasting the same attritbutes as Metal Mario.

Super Smash Bros. Brawl
The Metal Box item returns in Super Smash Bros. Brawl, and thus, allows Mario (or anyone else) to turn into a Metal form when the item is collected. This can also be used as a setting in the Special Brawl.

Metal Mario did not reprise his role as a boss.

Mario Party 7
Metal Mario also appears in Mario Party 7 when Mario uses a Metal Shroom Orb on himself to avoid traps. As a matter of fact, all the partiers could use the Metal Shroom Orb to make themselves metallic.

Mario Strikers Charged Football
It appears that Mario can become Metal Mario in Mario Strikers Charged Football when he performs his Mega Strike.

Trivia

 * In Super Smash Bros. Melee, near the place where the "M" logo would appear on his trophy is a view of the Yoshi's Island stage.
 * In an interview published in 1991 (in Nintendo Power's Mario Mania Player's Guide), Shigeru Miyamoto reportedly said: "Who knows what Mario will look like in the future? Maybe he'll wear metallic clothes!" Metal Mario then appeared in 1996.
 * In Mario Golf, Metal Mario's blue color change bears resemblance to Shadow Mario.
 * In the Super Smash Bros. series, Metal Mario appears unable to speak. Likewise, anyone else that turns into metal will lose their voice, except for Mr. Game & Watch.