He is the most trustworthy person in the moon empire. The moon empire is the bad side, ruled by Nightmare. On the opposite end is the sun empire, rule by nightmare's twin brother, Dream sans. Killer is in the moon empire. He has a crescent moon mark on his face which is tied to his soul showing that he belongs to the moon empire. If someone were to betray their empire it would greatly hurt them. Killer is obedient and abides all of the orders he is given by Nightmare.