Airi Akitsuki is the younger daughter of Kayoko Akitsuki and the stepsister of Marina Akitsuki, sharing no biological father with Marina. Her family dynamic changes when her mother marries Kouzou Akitsuki, a chemistry teacher with a doctoral degree at Airi's school. Initially, Airi harbors intense hostility toward her new stepfather, subjecting him to continuous verbal and emotional abuse over a prolonged period, resenting her mother's remarriage and refusing to accept Kouzou as family. She is a teenage girl with blonde hair styled in twintails, blue eyes, and a slender build. She attends school as a popular newcomer and forms a close friendship with classmate Sana Kuranaka, who bears a striking physical resemblance to her. Her interests include cosplay, though opportunities are limited. Kouzou, harboring illicit desires, develops an experimental aphrodisiac drug in his laboratory and uses it to overcome Airi's resistance, enabling repeated non-consensual sexual encounters. While initially resisting, she gradually develops a physical dependency and psychological attachment to Kouzou. Her feelings evolve into possessiveness and jealousy, especially when Kouzou pays attention to other girls like Sana or Marina. She openly declares Kouzou her exclusive sexual partner and actively competes for his attention. Her relationship with Marina includes sexual encounters, sometimes initiated through Kouzou's involvement, including three-way scenarios. Airi exhibits exhibitionist tendencies, engaging in sexual acts with Kouzou in risky public settings like train bathrooms, beaches, or ocean fronts. On one occasion, she orchestrates revenge against Kouzou by burying him in sand at a beach and deceiving a lifeguard into performing non-consensual oral sex on him while she watches from a distance. After engaging in sexual acts with Kouzou at a remote bus stop during heavy rain, she later denies the event occurred. Airi experiences pregnancy as a result of the non-consensual relationship. Later, she displays genuine affection and seeks authentic emotional connection, yet remains aware of Kouzou's continued use of aphrodisiacs to manipulate her. Sana Kuranaka also becomes a victim of Kouzou's drug and subsequent abuse after being brought to the Akitsuki home.