There are 8 different character types used throughout the entire game. All with different weighted stats.
(Male and Female specimens): Humans are the best fighters of the lot. Humans gain HP and abilities the fastest, their weapon skills are second-to-none, and can make equal use of both strength and speed weapons. They do, however, have a weakness to magic, both in the inability to cast it, and gain defences against it. The female gains HP slower, but gets Speed faster than the male. Both gain strength at the same rate.
(Male and Female specimens): Mutants are the equivilant of mages. They start the game with a hit-all magical attack (blizzard for the male, flame for the female), and gain mana quickly. They also gain enough speed to make use to any speed weapons you can find. Their strength, however, is low compared to humans, and strength weapons are all but ruled out. Give these guys a couple of spell books, and they're ready to rock. They gain hp slowly, but their speed help them go first in combat. The female gains speed faster than the male, while the male gets mana faster.
This is the oddball of the bunch. This guy dosn't level up like the humanoids, or change form like the monsters. He gains hp and stats by "upgrading" him, or equiping him. He is not restricted to just equip 1 helmet or gauntlet, but as many as he can hold. Try and keep him fully equipped, too. He often has the highest defence of the team, and even if he has less hp, put him in the front, as he will have about 5-10 more defence than the others. His guns are just as good as magic, as most take out a group at a time. This guy can't handle a spell, however, so be ready to cure during bosses.
(Slime, Red D, and Imp): The monster's starting class dosn't matter much, but if you really care, the Slime is a fighter, the Red D is an offensive mage, and the Imp puts the enemy to sleep. Anyways, these guys eat meat to "level up", changing form. If this form is for the better or for the worse, is unknown. THese guys, while possible powerhouses, are more often than not unable to dish out enough damage to get the job done. Worse still, they can become weak, first world creatures at the worst times.