Besides there being no Deadpool, you can't actually play as any of the characters. You select from three new characters to play and interact with the x-men and brotherhood, but you can't control any of them.
By the way, Deadpool isn't in the game because he is not part of the brotherhood or x-men, and the game deals with those two factions.
On a final note...don't get this game. rent it if you can. I bought it (stupidly, but huge x-men fan) and only have one achievement left after playing for 3 days (about 3 - 4 hours a day).