Kana Sumiya is a classmate of both Sora and Nami Aoi, and is the best friend of Nami, Sora's twin sister. Together with Nami, she is a member of their high school's cosplay club, where she is responsible for designing and creating the costumes they use. Kana is portrayed as a reserved and timid young woman, whose shy nature often prevents her from being forward about her personal feelings. Her personality is markedly more introverted compared to her friend Nami, leading her to be quietly observant rather than openly demonstrative.

Kana's primary motivation throughout the story is her romantic affection for Sora Aoi. She harbors these unspoken feelings for him but struggles to find the courage to confess them directly. This unrequited love for Sora places her in a complex position, as she remains largely unaware of Nami’s own deep romantic feelings for her, creating a subtle but persistent tension in their friendship. In her role within the narrative, Kana acts as a central figure in the romantic entanglements that complicate the existing dynamics between the main characters. Her presence is a catalyst for Nami's intense jealousy, as Nami is forced to watch Kana's affections be directed toward her brother rather than toward herself. This jealousy ultimately drives much of Nami's conflicted behavior and her volatile actions throughout the series.

The key relationships in Kana's life are almost exclusively defined by her connection to the Aoi twins. Her friendship with Nami is close but built on a misunderstanding, as Kana does not initially recognize that Nami's interest in her is romantic and not merely platonic. Her relationship with Sora begins with her quiet crush and eventually progresses to a sexual encounter; on one occasion, Nami even encourages Sora to sleep with Kana, an act she secretly films due to her own jealousy. A pivotal moment in Kana's development occurs when she accidentally walks in on a sexual encounter between Nami and Sora. Rather than reacting with simple shock, this event leads directly to a threesome involving all three characters. It is during this encounter that Nami's long-hidden romantic feelings for Kana finally come to light and are openly acknowledged. This marks a significant turning point for Kana, as she moves from being the object of unspoken desire to an active participant in a mutual relationship, and she and Nami subsequently begin dating. Throughout this progression, Kana demonstrates a quiet but growing capacity to accept and return the affections of her best friend, resolving the one-sided nature of their original bond. Outside of her interpersonal relationships, Kana does not possess any notable special abilities or unique talents beyond her skill in costume design for the cosplay club.