Kei Shindō is one of the central characters of the narrative, introduced as a first-year student at Otobane Academy in the town of Otowa. She lives next door to her childhood friend, Hiro Hirono, who is one year her senior. Kei is the identical twin sister of Chihiro Shindō, and the two sisters present a striking contrast in personality. Where Chihiro is quiet and reserved, Kei is outgoing, energetic, and highly competitive.

Physically, Kei is athletic and has a strong presence on the school's girls' basketball team, becoming a star player even as a freshman. She is also known for being a poor cook, a trait she shares with her sister, which has been a source of minor turmoil for those around her, particularly Hiro. Her daily routine often involves waking Hiro up for school, as he spends his nights drawing manga. This caretaking role has been a part of their relationship since childhood.

Kei's primary motivation in the early part of her story is her unacknowledged and deeply buried romantic love for Hiro. She harbors intense guilt stemming from an accident in their past. When the three were children, Kei and Hiro went to the beach without telling Chihiro. Chihiro, trying to follow them, was struck by a car and suffered severe injuries that led to her memory condition. Kei has always blamed herself for this event, believing that she is unworthy of Hiro's love because she feels he would have naturally ended up with Chihiro if the accident had not occurred. Consequently, she suppresses her true feelings and instead adopts a motherly, practical role in Hiro's life, helping him with chores and his work.

This dynamic is thrown into chaos when Hiro becomes involved with their classmate, Miyako Miyamura. Kei’s suppressed jealousy erupts, and she enters into a direct rivalry with Miyako. Her role in this love triangle is that of a fierce, desperate competitor who feels she is losing her preordained place by Hiro's side. Her actions become driven by a need to reclaim his attention, leading to conflict and an eventual, painful rejection when Hiro chooses Miyako over her. A serious injury sustained during a crucial basketball game exacerbates this period of loss, forcing her to confront giving up both the person she loves and the sport at which she excels.

Following this emotional and physical low point, Kei’s story shifts as she becomes the subject of an independent film being made by Kyosuke Tsutsumi, Hiro's friend and classmate. Kyosuke becomes infatuated with her image and persistently asks to film her. Initially seeing him as a substitute for the affection she lost, Kei slowly forms a new bond with him. Kyosuke, who has his own dreams of becoming a filmmaker, helps her see past her loss. Through their relationship, she learns to accept her past feelings for Hiro as something real and not wrong, finally allowing herself to move forward. She begins a romantic relationship with Kyosuke, returns to the basketball club, and heals her relationship with her sister and with Hiro and Miyako.

Kei’s primary ability is her athleticism, specifically in basketball. She is agile, quick, and a dominant player on the court. Beyond her physical skills, her most defining trait is her emotional resilience. While she suffers greatly from heartbreak and guilt, she ultimately demonstrates a powerful capacity to confront her past, accept reality, and build a new path for herself and her relationships. She is also known to have been the one who, in their childhood, encouraged Hiro to pursue his dream of becoming a manga artist, showing that her influence on those around her is significant and lasting.