He is a tall light yellow fish who has a purple ripple running through his body. He also wears plain clothes, a white shirt, and black pants. He looks just like Red, a tenant who stays at the Hotel Deep Six.
In The SpongeBob SquarePants Movie PC Game, a character who looks exactly like Marvin makes an appearance, except his name is Red and he has red skin instead of yellow. Also, he wears a blue shirt instead of a white one.