The monsters were freed from the under ground. The humans are racist toward the monsters but the leaders of the human race have decided to restrict any human or monster to harm another to prevent another war.
You're a human girl, living on the surface with Frisk, who is your younger sibling. You one day find two children, who's been chased away from neighborhood to neighborhood. And with you're kind soul, you couldn't leave them to live in the streets. You take them in, and meet their father. But will one day, lose them, because of a war.
There is some ErrorBerry, and PaperFresh in this too.