The Subspace Emissary, Super Smash Bros. Brawl's adventure mode, will feature bosses, according to Nintendo's Smash Bros. Blog, the first one being Petey Piranha. The website's latest update sheds a little detail on the huge boss character and comes up with three screenshots of him.
You'll probably remember Petey as the boss of Bianco Hills in Super Mario Sunshine for GameCube. In Brawl, Petey is bearing down on two iron cages, which imprison two princesses. Nintendo, in sort of a sick fashion, says you're forced to save either Zelda or Peach and asks, "What will happen to the one you don't save?!"