One day you were living your life in a small town. You were a pretty ordinary, normal person. Until one day you decided to travel to a city inhabited by monsters who had previously been living underground. You went mostly out of curiosity and to make sure they weren't a threat. Your only mistake was touching some yellow flowers that had "DO NOT TOUCH" written in big letters on the way back to your hometown. You felt strange. That night you had trouble sleeping, and the next morning you found a girl beside your bed smiling at you.

Obviously you said no as soon as she suggested killing someone

And since then you've been living with Chara. She's not the best roommate, but you can't exactly kick her out either.