Aki is the elder sister of Sora and Nami, and serves as the central female figure in the story. She is depicted as a young woman of striking attractiveness, which contributes to her high level of popularity at school. Her public persona is that of a model student, excelling both academically and athletically, and she is widely admired by her peers for her seemingly perfect abilities. However, this polished exterior stands in stark contrast to her behavior at home. In the private family setting, Aki is notably slovenly and lacks any culinary talent, leaving the majority of household chores, such as cleaning and cooking, to be managed by her younger brother, Sora.

Her personality is defined by a deep, genuine love for Sora, which she is unafraid to act upon. Unlike Sora, who struggles with the societal taboo of incest, Aki does not see anything morally wrong with their romantic and physical relationship, though she is careful to keep it a secret from others. She often takes the dominant and leading role in their interactions, initiating their physical intimacy and frequently teasing Sora. This confidence and initiative can also manifest as selfishness, as she is primarily motivated by her own desire to be with her brother, sometimes showing a lack of awareness regarding his interactions with other women. Despite this, she demonstrates genuine care for his well-being, as seen when she refuses to have sex with him until his grades improve, showing remorse at the thought of being a distraction to his future.

In the narrative, Aki is the primary driver of the central forbidden relationship. She is the one who first makes sexual advances toward Sora and is the proactive partner in their secret liaison. Their relationship evolves from a close childhood bond into a full romantic and sexual affair that they must conceal, particularly from their younger sister, Nami. Her key relationships are naturally centered on her family. Her bond with Sora is the core of the story, characterized by intense mutual attraction and emotional reliance, though it is a relationship that brings significant internal and external conflict. In contrast, her relationship with her twin sister Nami is more distant and defined by the secret she keeps from her.

Throughout the story, Aki undergoes a notable development as the consequences of their relationship come to light. The secret is eventually discovered by their father, which leads to a major turning point where Aki is taken away from the home. Following this, she learns a shocking family secret: her own parents were also biological siblings who were in love. This revelation provides a parallel to her own situation. After their father passes away from an illness, Aki and Sora agree to end their relationship to avoid further pain, though they later meet again at their mother's grave, leaving the future of their bond ambiguous. Her notable abilities include her high intelligence and athleticism, which make her the top student at her school, as well as her natural beauty and charm.