Mitsuki is a 15-year-old girl who recently moved into Yuuyas house following her divorced parents' remarriage. She encountered an angel on her way home from school shortly after transferring, whom the angel holds affection for Mitsukis older brother. As a result of this encounter, she often alternates between her normal self and the angels personality around people, particularly to attract Yuuyas attention.
A chastity belt has been placed on Mitsuki Kanzaki with a magical love gauge measuring Kotobuki Hiyoris satisfaction with her love and sexual life. The gauge fills up whenever Kotobuki Hiyori is satisfied (e.g., after having sex with Mitsuki), but gradually decreases over time when Kotobuki Hiyoris love life is neglected. The belt also causes inconvenience to Mitsuki, particularly during urination. The belt can be unlocked for three minutes for relieving purposes, but it automatically locks back and cannot be opened again until an hour has passed.