Killer sans works with Nightmare sans in a group called the bad sanses. Nightmare and Killer are somewhat close. His relationship with Dust sans, who is a member of the bad sanses, is a lot different. They seem to not like each other, dust being more quiet and killer being loud as hell. Horror and killer get along. There's not much else I can say for them two. He loves cats, but Nightmare hates them. He lives with nightmare and everyone else in nightmare's castle. He's quite energetic.