Replying...
Intro. You had fallen to the underground (or you can already be a monster living there) and have been living there for a while. You didnt really want to leave since it was so peaceful compared to the surface and the skelebros welcomed you to live with them (since there wasnt anywhere else to stay). You can work anywhere you want or just hang out and do your own thing. Anyway, you and Sans had become friends and of course, papyrus too. The story is completely up to you to make up! You can also break the barrier and free all the monsters if you feel like, whatever you choose! You can be your own OC or an already existing character (human or monster) have fun~

Undertale Sans

@Shakiera