Sebastian’s body is shaped similarly to that of a sea snake, in that it is elongated with blue scales. At the end of his body is a large tail fin. Sebastian’s skin is a light blue. His torso is humanoid, with two larger arms and one smaller arm on his left side.

Sebastian’s hands are humanoid, although he only has four fingers with claw-like ends. Sebastian’s face retains a humanoid face (which is mostly always smiling) with fish-like features, such as an anglerfish bulb on his forehead.