The Skeleton Girl

She lived alone, her name unknown, in an unknown place, in an unknown and forgotten city. People knew not to go there, she would chase them out. Because of what she wears, she is known among nearby societies as The Skeleton Girl.

He is Gaster!Sans, preferably just sans, and he hunts humans. An assasin for the monsters in the war. He recieves intel about someone who lives alone in an otherwise uninhabitted city from a human in exchange for him sparing them. When he goes to investigate, he finds it harder than he thought to catch the mysterious Skeleton Girl.