As many on the forums guessed and/or feared, Petey Piranha, originally appearing in Super Mario Sunshine, is the first boss character announced for Super Smash Bros. Brawl.  In the earlier Subspace Emissary (Adventure Mode) video, Petey had locked up both Peach and Zelda in cages and Kirby was ready to face off against the newly-fearsome monster.  The update says that you can only save one of the women...

Link: Smash Bros. DOJO!!