I'm going with Rosa. This was one of the few Final Fantasy games where buffs and debuffs were needed. Slow and Haste especially; smurf you Demon Wall, smurf you! Plus keeping everyone else from dieing was pretty nice too. She even brought them back when the enemies cheated and killed them somehow. Also with the Augment system in DS she is mean! Dualcast+Pierce+Omni Casting means you can have Reflect and Haste up on everyone in the same turn. And you can keep healing them with Reflect up.