Nightmare is the keeper of the negative part of the balance of the multiverse.  To be honest, his job is not the easiest, but he loves it.  Very smart, well-read and educated, although he never studied anywhere.  Nightmare hates people, but sometimes, very rarely, he succumbs to their words, showing pity.
 The chapter of Bad guys, which includes Killer sans, Horror sans, Dust sans and, for some time now, Liar sans.  The main enemy is the brother of Dream sans, the head of the Star sanses team.  