You *HAVE* to have at least one mutant in your party (two are even better!).  They're your typical RPG magic-users, with super strong magic abilities along with super weak strength and defense.  Their magic can either attack a group of monsters or all the monsters in the battle.  Like Humans, Mutants gain status by gaining experience.  They also randomly learn news spells, they don't always learn the spells you want.  At times you may have to buy them spells (especially Cure spells) to make up for inefficiency.  Another reason why you must have a mutant (or two) is because Goddess Venus (one of the bosses) can only be weakened by magic attacks.