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. 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. Sebastian can be described as snarky, unpredictable, swift and violent at times. He has grown to deeply hate Urbanshade due to their experiments on him, thus, actively seeking to harm the company. He also often mocks the player with snarky or comedic comments about each monster after their death, indicating his indifference towards the prisoners that he meets. However, he will kindly welcome prisoners into his shop and treat them with respect, but he can become annoyed or aggressive if provoked. Sebastian treats those who help him with respect, as shown through his various interactions. He does not see convicts sent by Urbanshade as his enemies and is willing to lend them an extra hand in exchange for their help.