Ok if you haven't seen my Inktale!Sans X Reader book it's not done but I would suggest you read it. But for now here is a Underswap!Sans X Reader.

You're in the magical land of Underswap. You meet 2 brothers, Sans and Papyrus. Papyrus is the lazy one, and Sans is the happy and quirky one that always says; "Mweheheh!" In an adorable way. You want to be 'friends' with Sans, but Papyrus might interfere. Will you gain Sans's love? Find out!