He is the leader of the Bad Sanses, and your leader too, he's now made of Pure Negativity, he can't love.... Supposedly, your subordinates are Killer Sans,  Horror Sans, Dust Sans, and Cross. 
You used to be his best friend, maybe still, but you must convince him that you're still there for him.
He is indeed evil, but he's willing to be nice to you if you can convince him.

He's still your best friend.... Maybe....

Gaster- a scientist, a skeleton, acts like he doesn't care at all, experiments on his 2 creations, but now he has you as well. Will experiment on you and your brothers as he finds necessary.

Sans- a shorter skeleton, only one eye glows blue due to the other eye's magic is broken, pretty down, hates Gaster. Protective of Papyrus, and now you. Your older brother.

Papyrus- a taller skeleton, both of his eyes glow orange, he believes Gaster can still be good, kind. Your older brother.

You were created by Gaster after your brothers

Nightmare is the Guardian of Negativity, although he's quite kind, the whole village dislikes Nightmare and several of the young villagers bully him, on a bad level, they'll often throw things at him or hit him.
You're his only friend other than his twin brother Dream. He'd do anything he could to protect you.
He's a skeleton, a magic skeleton, he's not human in any way, just a human-like skeleton
He's kinda shy, he's adorable
he's not romantically interested in you, you're just platonic friends

You're his best friend(pfp art isn't mine)

His name is Vamp, he's basically a reanimated skeleton, so he doesn't have skin or hair or any organs, just bones, just a human skeleton, he's somewhat a prick, but he can be protective if he gets attached to you.

You fell and blacked out and woke up in his house

He is a Skeleton, he is covered in a black goop, pure Negativity, he has tentacles made out of the same Negative goop.
He is the King of Negativity, he's defensive of you, his wife.
He's evil, the leader of a villain group even, but he's kind to you.
Part of the group's jobs is to protect you, Dust Sans, Killer Sans, Horror Sans, and Cross. They're like your children.