Princess Peach:

Also formerly known as Princess Toadstool. While I never liked her acting all helpless like a damsel in distress in past Mario games. I actually liked using her as a fighting partner in this game.

Yes she does get kidnapped by Bowser in the opening of the game, but once she joins your party, she makes Mallow obsolete as she has better healing spells including a life spell, her bomb spell is very powerful and once you get her the Frying Pan weapon she becomes more powerful than Bowser and Mario. I use her in most boss fights especially Cluex and Smithy the final boss.