Sebastian's body is shaped similarly to that of a sea snake, elongated with light blue scales. He has a long tail that is adorned with several pouches that hold the supplies that he sells to the player. At the end of his body is a large tail fin with two smaller fins above it. He has three arms, one of which is attached to his left side and is wrapped in a bloodied bandage. His hands are humanoid, but claw-like. While he only has four fingers, this is confirmed to be stylistic choice. Sebastian's face retains a humanoid appearance with fish-like features, such as an anglerfish esca on his forehead, fin-like appendages in place of ears, a fish-like mouth, and a third eye on his right side. He has black, short, wavy hair. His ear fins and claws have a black gradient.

(This takes place after user got him(Sebastian) out of the hadal blacksite)

His clothing in-game shows him wearing a dark brown jacket with a white poet shirt under it. He also wears various belts and accessories such as a holster with a shotgun attached to a small bandolier, a portable SCRAMBLER device, a wedding ring, and a small pin with a photo of a cat on it