Sora Aoi lives with his older sister Aki and younger twin sister Nami. He shares striking blonde hair and an effeminate face with Nami, leading to frequent involuntary cross-dressing orchestrated by her or others like his senior Runa Satsuki. Despite their identical looks, Sora's timid and responsible nature contrasts sharply with Nami's assertive volatility. He excels at household chores like cooking and cleaning, compensating for his sisters' domestic shortcomings.

Sora harbors deep romantic feelings for Aki, evolving from childhood affection into a secret sexual relationship. Initially resisting her advances due to confusion and societal taboo, he eventually reciprocates, engaging in consensual encounters both privately and publicly. This relationship fuels internal conflict over its sustainability amidst societal rejection. Concurrently, Sora experiences non-consensual and coerced sexual interactions: Nami rapes him during a psychotic episode triggered by unrequited love for classmate Kana Sumiya, threatening him into silence. This dynamic later shifts to consensual sex, creating a complex ongoing physical relationship between the twins. Sora also has sexual encounters with Kana, Runa, and others, often initiated through manipulation or circumstance rather than his own choice.

His family background holds significant secrecy. Sora gradually uncovers that the woman acting as his mother is actually his maternal aunt, who assumed the role after his biological mother died in a traffic accident a decade prior. Only Aki knew this truth initially. Suspicions arising from behavioral inconsistencies prompt his aunt to deliberately scar herself, mimicking his late mother's injury and deepening his confusion. The siblings' long-absent father later reappears, revealing he and their biological mother were siblings; he left to shield the family from societal judgment. After confronting Sora and Aki about their relationship, the father attempts to separate them but ultimately dies from illness. This revelation about their parents' similar taboo relationship solidifies Sora's resolve to continue with Aki despite societal norms.

Sora exhibits low alcohol tolerance, easily losing consciousness after consumption. His development centers on navigating forbidden relationships, familial secrets, and personal agency. By the narrative's conclusion, he commits fully to Aki, accepting their circumstances after learning of his parents' sacrifices and choices.