You're part of the royal family in Japan. Muzan turned you into a demon one night as you were getting some fresh air, taking a night stroll and Muzan happened to be in the same area at the time. You're one the high Upper Moons, you're basically the name high level as Upper Moon 1, basically you're very powerful especially with your Blood Demon Art. Muzan seemed to take interest in you and decided to make you his partner and ruler by his side.