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Yuni Irido is Yume's mother. After divorcing her first husband, she remarries Mineaki Irido and becomes Mizuto's stepmother. Mineaki Irido is Mizuto's father. He remarries Yuni, thus bringing Mizuto and Yume together. Akatsuki Minami is Yume's best friend. Kogure Kawanami is a childhood friend of Akatsuki. The plot revolves around how former lovers learn to coexist in a new, related role, overcoming awkwardness and internal contradictions.

Isana Higashira Isana is Mizuto's classmate and close friend. Appearance: She has short green hair and green eyes. Personality: Isana is a shy otaku girl with a genuine passion for light novels. She struggles academically, but manages to enroll in the same high school as the other characters. Through their shared passion for books, she quickly becomes close to Mizuto. Story: Isana falls in love with Mizuto, and with the support of Yume and another friend, she decides to confess. However, Mizuto rejects her, explaining that he doesn't want to upset his sister, Yume. Despite this, Isana doesn't become embittered, but remains friendly with him.

Yume Irido (née Ayai) Yume is the main female character, Mizuto's ex-girlfriend, who became his half-sister after his mother's remarriage. Appearance: A pretty girl of average height with long, straight black hair and brown eyes. In middle school, she was more withdrawn: she wore glasses, simple clothes, and braided her hair. Personality: In middle school, Yume was shy and a big reader, but when she entered high school, she consciously decided to change: she became an excellent student, began to follow fashion, and learned to socialize. However, her "perfect student" image sometimes falters, especially when it comes to Mizuto. History: Before her mother's marriage, she dated Mizuto, but they broke up. Soon after, her mother married Mizuto's father, and Yume found herself living under the same roof with her ex.

Mizuto Irido Mizuto is the main male character, a high school student with a high intellect and a deep love of books. Before the events of the series, he was a withdrawn otaku, indifferent to the opinions of others. Appearance: He has brown hair and brown eyes. He wears comfortable clothes and doesn't take much care of his appearance, although with proper grooming he can look attractive. His childish face is often called cute. Personality: Despite his outward apathy, Mizuto is caring: he supports Yume when she gets into awkward situations and shows tenderness towards her. He is a good student and competes with Yume for first place in the class. History: He and Yume dated in middle school, but broke up shortly before graduation due to escalating differences. Soon after, their parents married, and the former lovers became stepbrother and sister. studies in the same class with Yume, and sits right at the desk in front of her.