Metal Bros.

The Metal Bros. appear in Melee's Adventure Mode as one of the stage bosses.

Metal Mario
Metal Mario more or less reprises his role as a special opponent in a single player mode, in this case as an opponent in the level of the Adventure mode taking place on the Battlefield stage, following the battle with fifteen Fighting Wire Frames. Unlike the last time, Metal Mario is not a unique opponent; he is actually a Mario that is permanently under the effect of the newly introduced Metal Box item, except he falls much faster. This makes him more aggressive, and his recovery is longer, but the metal does not increase his weight as much as it used to. If the player has unlocked Luigi, Metal Mario teams up with Metal Luigi for that same battle, making it more difficult.

Mario can also become Metal Mario by using a Metal Box.

[edit] As a trophy Metal Mario also appears as a trophy which can be acquired randomly through the 1-P mode or the Trophy lottery:

Metal Mario "Mario was in for a surprise the first time he hit a Metal Box in Super Mario 64. The metal cap that popped out transformed Mario into living metal, giving him great stamina, a heavy tread, and the ability to walk underwater. In Super Smash Bros., Metal Mario showed up as an incredibly stubborn midlevel boss." (Super Mario 64, 9/96)

Metal Luigi
Metal Luigi appears in Super Smash Bros. Melee with Metal Mario by falling from the sky of Battlefield in Adventure after the Fighting Wire Frames are defeated. He only appears if the player has already unlocked Luigi. Stat-wise, he is like Metal Mario - very heavy and fast falling - except he shares his attacks with Luigi instead of Mario. He also appears with other Metal characters in Classic Mode in Brawl.