Most of the Big avengers Including Captain America, Hulk, Wasp, Iron Man and Tigra are Available. Some X-Men are Shadowcat, Wolverine, Emma Frost( really kicks Ass), Jean Grey and Gambit and Cyclops.
Other Heroes included are Magik, Cable, Havok, Power Man & Iron Fist, Valkerie (Really kicks Ass)Quicksilver and many others.
The only one unavailable to me is Mockingbird. She was a speciak acquisition early in the game before I could do the special mission where she was the reward. Coming soon as PC's are X-23. Magneto & Punisher. http://avengersalliance.wikia.com/wiki/Category:Heroes