Mitsuki Kanzaki is the main protagonist of the anime. She is a high school freshman whose life is disrupted when her mother remarries a man with a teenage son, Yuya. Shortly after the marriage, her new stepfather is transferred overseas and her mother follows him, leaving Mitsuki to live alone with Yuya. Mitsuki has a history marked by absent and abusive father figures, which makes her slow to trust and reluctant to accept her new stepfamily. She initially keeps a cold distance from Yuya, rarely speaking to him or showing any warmth.

Her personality is initially reserved, guarded, and somewhat sour. She feels abandoned by her mother, and this sense of betrayal fuels her standoffish behavior. However, the supernatural events that follow force her into an uncomfortable closeness with Yuya. One day, Mitsuki is possessed by the spirit of a young girl named Hiyori, who died before confessing her love to Yuya and cannot reach heaven. Hiyori places a chastity belt on Mitsuki that fills up whenever Mitsuki experiences sexual feelings toward Yuya; each time it fills, Hiyori takes a step toward heaven. Because failing to help Hiyori would result in both their deaths, Mitsuki is compelled to make bold romantic advances on her stepbrother against her own will.

Mitsuki’s central motivation shifts from simply wanting to get rid of Hiyori to grappling with the authenticity of her own emotions. As the series progresses, she begins to question whether the feelings she develops for Yuya are truly hers or merely a reflection of Hiyori’s lingering love for him. This internal conflict drives much of her character development. She evolves from a closed-off, resentful girl into someone who must confront her changing feelings and her own capacity for love, even in the bizarre circumstances.

Her key relationships are with Yuya, her stepbrother and primary romantic target; Hiyori, the ghost who shares her body and whose desires sometimes override Mitsuki’s own; and her mother, whose departure leaves Mitsuki feeling neglected and shapes her initial distrust. Mitsuki’s development is marked by her growing ability to open up, her occasional moments of genuine affection, and her struggle to distinguish her own desires from Hiyori’s influence.

Notable abilities are not supernatural in Mitsuki herself; she is an ordinary high school girl. Her most distinctive trait is that her body becomes a vessel for Hiyori, and she is subjected to the chastity belt’s magical condition, which forces her into repeated intimate situations. She also shows a quiet resilience in dealing with the absurdity of her predicament, and over time she learns to navigate the complicated emotional landscape created by the possession and her evolving bond with Yuya.