Aki Aoi is the older sister of Sora and Nami Aoi. She embodies a stark contrast between her public and private selves. At school, she is the model student: athletic, academically accomplished, and highly popular, her reputation built on perceived intelligence and social grace. Domestically, however, she shows significant shortcomings, lacking culinary ability and failing to maintain basic cleanliness in her personal space, often relying on Sora for tasks like cooking.

Her character arc centers on a secret incestuous relationship with her younger brother Sora. She initiates their physical intimacy, first unsuccessfully in a shower encounter where Sora flees out of embarrassment, then successfully in his room following her confession of romantic feelings. This establishes a pattern of repeated sexual encounters in diverse locations, including the school gym storage, an onsen, a Ferris wheel, and a love hotel. During these moments, she insists Sora avoid calling her "elder sister" (nēsan), preferring direct address, though Sora struggles with this. Her sexual assertiveness and high libido are recurring traits, and she appears unaware of Sora's non-consensual or coerced encounters with others like Nami or Kana.

Her relationship with Sora drives significant emotional development. When his academic performance declines due to their frequent liaisons, she voluntarily abstains from sex until his grades improve, expressing guilt over potentially hindering his future. She exhibits jealousy observing Sora's growing closeness with their twin sister Nami, prompting her to orchestrate an out-of-town trip to rekindle their bond. After initial rejection during the trip, they reconcile romantically. Later, after their father – revealed to have engaged in a sibling relationship with their deceased mother – discovers their secret and separates them, Aki chooses to leave Sora, believing ending the relationship will protect him from further emotional harm. The narrative concludes with their reunion at their mother’s grave, implying a reaffirmation of their connection.

Physically, Aki distinguishes herself within the family; unlike her blond-haired siblings and aunt, she possesses brown hair. This trait visually underscores her role as both an insider and an outlier within the family dynamic. Her characterization consistently intertwines her external perfection with private emotional conflicts, particularly her guilt-ridden yet persistent devotion to Sora.