Complete the task to unlock the character:

Abomination - Defeat Abomination.
Grey Hulk - Smash 10,000 objects.
Hulkbuster Iron Man - Have a saved game file from Iron Man.
Ironclad - Defeat the U-Foes, trick a U-Foe into KOing his teammate; or defeat Abomination.
Maestro - Destroy a building in each neighborhood.
Professor Hulk (Smart Hulk) - Complete all jump challenges.
Savage Hulk - Collect five comic book covers.