You're in a world post-game of Undertale. Monsters were flooding up from the Underground, and no one was quite sure what to do with them. Most people knew of them, and expected it eventually. It was hard to get used to seeing these... People, you guess, but it became normal after the first month or so. They started getting jobs around, buying houses with their currency (which was Gold, and due to inflation of the new paper currency it was worth insanely more than normal money) and most of them were well off with their now nice lifestyles of upper middle class monsters.
You're a simple girl, very short (barely under five feet), with a curvy figure and just over shoulder-length dark brown hair. You have a stupid job and a horrible college budget that made life kind of hard to do alone in your three-room apartment, which was the only thing close enough to the campus to get. It was meant for two people and a guest, but you had no one to move in with you. So, you put out a Cruggs' list ad asking for at least one roommate, maybe two.
One person answered.
Two monsters stepped into your life, and you have no clue how important these brothers were going to be to you- How important the always-smiling, pun-shooting monster was going to be to you.
(Majority of the characters you're introduced to belong to Undertale. Writing by me, your character loosely based off myself, but only physically [height, gender, body type and size in general], the mental personification is just whatever happens.)