These are are the Main Characters in the game:
Bishop
Colossus
Cyclops
Deadpool (unlocked after completed first time)
Gambit
Iceman
Iron Man (unlocked with Homing Beacons)
Juggernaut
Magneto
Nightcrawler
Jean Grey
Professor X (unlocked after completing all Danger Room mission)
Rogue
Scarlet Witch
Storm
Sunfire
Toad
Wolverine
Here is a list of who you meet that isn't playable:
Act I: Sabretooth
Mystique
Blink
Angel
Beast
Pyro
Blob
Act II: Forge
Destiny
Ka-Zar
Shanna
Act III: Vindicator
Guardian
Havok
Emma Frost
Department H operative
Act IV: Shadowcat
Sebastian Shaw
Black Queen
Banshee
Moira MacTaggert
Nick Fury (only on communicator)
Sergeant Kilroy
Act V: No one new Here is a list of all the bosses:
Act I: Zealot
Grizzly
Lady Deathstrike
Abyss (FINAL)
Act II: Garokk
Sauron
Omega Red
Mikhail (FINAL)
Act III: Sugarman
Stepford Cuckoos
Holocaust (FINAL)
Act IV: Bastion
Stryfe
Archangel (FINAL)
Act V: Mr. Sinister with Beast
Apocalypse (FINAL)