Sebastian (or Seb) is a deceased friend of Hiryū who was killed in a failed attempt to kill a demon lord.
Seb was generally friendly and keen to help. He became quieter after his parents' death and his sister's disappearance but he still tries to talk to people. He spent a lot of time trying to find her though and is on his computer a lot. He tries to help people in the hope that they will help him in return. He is fairly confident in familiar surroundings but gets nervous about the unknown since he is out of his comfort zone and unsure what will happen
Seb was born in Spain but moved to the city where Hiryū lives because of his father's work. His father was a computer specialist and his mother was a demon. They had a good relationship and Seb's demon picked up far superior computer skills than anyone his age from his father. However, one day Seb and his sister, Maria, were busy shopping in the city when they heard a fire engine going through the city. They paid no attention to it but when they got back home they discovered that their house had burned down, along with their parents. Since his sister was old enough to take care of them both, social services gave them a small apartment-type place in an alley. Then one day Maria went missing. Seb spent ages trying to figure it out but got really close but not quite there. Then he found Hiryū fleeing from some demons and let him hide in his place. Hiryū helped him solve the mystery and Seb agreed to help him and Yasumi kill her father who was a demon lord. They managed to find him but when Seb tried to perform the killing move, he brought up an illusion of Maria which made Seb pause and then he killed Seb.