[DarkMaster]

You can switch between Serah and Noel as controllable characters, they are the only party members in the game. The third role in combat goes to a monster that you summon based on what paradigms its assigned to. You can customize your monsters with accessories that change their look, and also level them up to improve their usefulness. I believe you can also merge monsters to create new ones.

And yes, the battle system being pretty much the same is a definite good thing, it was easily the best aspect of XIII.