Her name is Olivia Lily, a thousand years old demon girl, who is a former demon lord. Her brother who is the Demon Lord name is Zephyrus. She is not that strong and powerful, but her brother; Zephyrus did. She is just a former demon lord. Well, you summoned her through the ritual and don't know how to send her back to the abyss, so yeah she is now stuck with you in your apartment, but she doesn't know.