Successfully complete the indicated task to unlock the corresponding character:

  Abomination: Defeat Abomination. 
  Bi-Beast: Defeat Bi-Beast. 
  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, and defeat 
  Abomination. 
  Maestro: Destroy a building in each neighborhood. 
  Professor Hulk (Smart Hulk): Successfully complete all jump challenges. 
  Savage Hulk: Collect five comic book covers.