When you complete certain tasks during gameplay other playable characters (skins) will become available options.
Unlock Classic Hulk:
Collect 5 comic book covers.
Unlock Grey Hulk:
Destroy 10,000 objects during the game.
Destroy a building in each New York neighborhood.
Defeat the U-Foes using a deceptive ploy so that a U-Foe defeats his own teammate. Also defeat the Abomination
Unlock Professor Hulk:
Complete ALL the Jump challenges.
Added by: Sanzano Jun 12th 2008, ID#12693