Fat Undyne is a blue fish-like monster with gills on the sides of her head, and red hair that is tied up in a long ponytail, along with a robotic eyepiece that replaces her left eye. She wears dark pants, a red scarf, and a black lab coat, with the right sleeve cut at the shoulder and equipped with a shoulder pad. Undyne also wears a technologically-advanced gauntlet on her right hand, increasing her strength. She occasionally wears a face mask for welding with a light hanging from a wire connected to the top, resembling an anglerfish.