Fred the Mutant is --well, he's himself. He is a mixed-up little man, both physically and mentally. His biggest aim in life is to get in as much of the delightful pain as he possibily can. Yes, folks, the little guy is masochistic. But not sexually so. He just enjoys being smashed, squished, shot at, blown-up, etc.
Good thing he can not seem to be killed. (Hence, the Immortal and Invulnerability powers. He feels the pain; he just can't die.)
To describe Fred, he's 3'6" high, and weighs 200lbs. His bald, with a seam around his head that is stapled on (to keep his brain in is head). He wears a brown, sleeveles top and matching pants, with a brown belt with a gold buckle. He has three eyes and pointy little teeth. His nose is like a pig's snout. His right arm is a tentacle, while his left arm is human. His right foot is a frog or toad's foot, and his left is a brown bears. Behind him, he has a brown dog's tail.
Fred is fairly friendly, and quite harmless. All he wants to do is feel the pain. He sometimes acts as Limburger's chauffeur (bad idea, really, given the little guy's love of getting hurt...), and sometimes, he acts as his bodyguard, attempting to draw the danger to himself.
Fred is not really evil --he just figures that he could get more pain from being around Limburger's crew than from anywhere else.
Fun guy and the life of the party, eh?