By completing certin feats unlocks other playable characters (skins).

Character	How to unlock
Abomination	Defeat Abomination.
Classic Hulk	Collect 5 comic books.
Gray Hulk	Smash 7500 objects.
Ironclad	Defeat the U-Foes, trick a U-Foe into KOing their team mate, and defeat Abomination.
Maestro	Destroy a building in each neighborhood.
The Professor	Complete all jump challenges.