Mizuha Kiryū, adopted into the Kiryū family following her biological parents' death in childhood, is approximately five months younger than her adoptive brother, Keiki Kiryū. Born on March 3, she stands 157 cm tall with an attractive appearance featuring neck-length light brown hair and blue eyes. She typically wears her school uniform but changes into casual clothing at home. Presented initially as an ideal younger sister—kind, cheerful, and attentive—Mizuha often seeks head pats, sneaks into Keiki's bed, or offers him advice. This normalcy conceals significant complexities, including intense romantic feelings for Keiki rooted in a childhood promise he made to always be by her side after her parents' death, forming the basis of an obsessive attachment. Her psychological development includes a pronounced exhibitionist tendency originating from a fifth-grade incident where forgetting her panties after swim class led to an unexpected sense of liberation. This resulted in habitual behaviors like attending school without underwear and secretly taking provocative self-photos to recreate that feeling. Despite this largely concealed tendency, she maintained a relatively normal reputation among peers, was popular with boys, and rejected all confessions. Mizuha's role involves being revealed as "Cinderella," the anonymous sender of a love letter paired with her panties to Keiki, discovered during his search to identify the sender. She later joins the Calligraphy Club to monitor and compete with other female members interested in Keiki. Her romantic pursuit becomes overt after revealing her feelings and lack of blood relation, initiating a date and sharing a first kiss with Keiki, culminating in marriage by the light novel series' conclusion. Key relationships define her character. Her dynamic with Keiki evolves from seemingly platonic interactions to openly acknowledged romantic tension. She maintains superficial cordiality with Calligraphy Club members while harboring rivalry over Keiki, but resents the Student Council for reducing Keiki's time with her and suppressing her exhibitionist urges during club oversight. Mizuha exhibits subtle yandere traits, reacting with quiet anger to Keiki's intimate interactions with others and making possessive declarations like insisting only his sister should pamper him. Her strategic romantic approach leverages societal acceptance of non-blood-related sibling relationships, rationalizing the cultural popularity of "imouto" tropes to her benefit. Her character develops from concealed desires to proactive romantic agency, navigating social taboos while reconciling her exhibitionist tendencies within her committed relationship.