Metal Mario is an metallic version of Mario who has appeared in Super Mario 64, two Mario Party games, a golf game, a tennis game, Dr. Mario 64 and Mario Kart 7, in addition to all three Super Smash Bros. games, mostly as a mini-boss.
As a mini-boss
Metal Mario appears in the 1P Game mode of this game as more or less a boss opponent to be battled one-on-one. Despite appearing to be just a simple palette-swap of Mario, he is a very different fighter; he barely flinches to ordinary attacks and is almost unaffected by throws at low percentages, and can sustain an absurd amount of damage before even being put into KO range (often over 300%). However, he is very weak to edge-guarding: once a player manages to knock him off the stage, he will often fail to recover due to his extremely fast falling speed, despite the decent distance that Super Jump Punch grants. Metal Mario never sprints and rarely jumps; he simply walks from place to place at a steady pace. Metal Mario also seldom uses his fireball even though a normal CPU Mario uses it often. On lower difficulties he mainly walks around the stage, seldom attacking, but he can be tougher on higher difficulties.
As a playable character
Metal Mario can be controlled using certain cheat codes such as the debug menu. However, many players consider Metal Mario to be far worse than normal Mario; despite his aforementioned strengths in resisting knockback, he has reduced combo potential, as he cannot efficiently follow up with combos after attacking due to an abysmal short hop, and has awkward aerial and dashing physics. As mentioned before, he also has extreme falling speed which not only hinders his recovery, but makes him very vulnerable to chaingrabs and certain combos.
As a mini-boss
Metal Mario more or less reprises his role as a special opponent in the single-player Adventure Mode. He appears on the Battlefield stage after the player defeats the fifteen Fighting Wire Frames. Unlike the previous game, Metal Mario is not considered by the game to be a separate character, but rather regular Mario permanently under the effects of the new Metal Box item and with slightly faster falling speed than usual. He also does not resist knockback as much as in the original game. If the player has unlocked Luigi, Metal Mario teams up with Metal Luigi for that same battle, making it more difficult. Beating both characters will award the player with the Metal Bros. KO.
In addition to his appearance in the Adventure Mode, there is a chance that Metal Mario can appear as an enemy in the penultimate fight of the Classic Mode, which pits the player against a random metal character.
As a trophy
Metal Mario also appears as a trophy which can be acquired randomly through the 1-P mode or the Trophy lottery:
Despite his role as a boss in Smash 64 and Melee, he, alongside Race to the Finish, didn't return in Brawl. In Level 8 of Classic Mode the player will fight one of several combinations of Mario characters with random modifiers, but while "Metal Mario" is indeed one of the random possibilities, he is treated as a regular matchup. However, Mario can still become Metal Mario by using a Metal Box or battling in Metal Brawl.