Kei Kishimoto is introduced as a fifteen-year-old girl who attempts suicide by slitting her wrists in a bathtub, a decision driven by the immense stress and pressure from her mother to excel academically and achieve high test scores. This constant bullying and comparison to her younger sister, who surpasses her in all areas, leads Kishimoto to feel she can never gain her mother's favor, pushing her to end her perceived failure. However, the mysterious black sphere known as Gantz intervenes, creating a perfect copy of her at the moment of death. Unbeknownst to the system, the original Kishimoto survives and is taken to a hospital, leaving the copied Kishimoto in a unique and impossible situation where two versions of herself exist simultaneously.

In terms of personality, Kishimoto is gentle, kind-hearted, and possesses a notable naivety, especially concerning boys and social situations. This innocence is highlighted when she asks Kei Kurono to be her "pet" and then sleeps in the same bed as him, completely unaware of the implications or the wrong ideas he might get. She forms a strong emotional attachment to Masaru Kato, who protects her from a yakuza member during her first, disorienting moments in the Gantz room. This act of kindness leads her to fall deeply in love with him, a devotion that becomes a central part of her character. This often puts her at odds with Kurono, who develops feelings for her that she does not reciprocate, and she unintentionally frustrates him by constantly talking about Kato.

Kishimoto's primary motivation shifts from survival to protecting Kato. After her first mission, she discovers she is a copy and cannot return to her original home, leaving her with nowhere to go. She ends up staying with Kurono, but her heart remains fixed on Kato, whom she sees as her true love and savior. Her role in the story is significant during the first three missions, where she serves as a central figure whose relationships with the two male protagonists drive much of the early character drama. During the Tanaka Alien Mission, she proves her capability by using the Gantz suit and weaponry to kill two aliens, earning ten points.

Her key relationships define her arc. With Kurono, she shares a complicated dynamic where he desires her but she only sees him as a friend, leading to tension and her eventual departure from his apartment after he becomes angry with her obsession over Kato. With Kato, her bond is one of pure admiration and love. This relationship reaches its climax during the Buddhist Temple Alien Mission. When the powerful Thousand-Armed Kannon alien launches a corrosive liquid attack at Kato, Kishimoto selflessly throws herself in front of him, sacrificing her life to save his. In her final moments, she confesses her love for him, solidifying her development from a suicidal, confused girl into someone who finds purpose and meaning in giving her life for another.

Kishimoto's development is a clear journey from despair to selflessness. She begins as a victim of her circumstances, attempts suicide, and is thrown into a violent world she does not understand. Through her experiences and her love for Kato, she finds a reason to live and fight. Her death is a pivotal moment, showcasing her complete transformation. As a participant in the Gantz game, she is equipped with the standard black suit that enhances her physical abilities and is armed with weapons like the X-gun, which she famously names for its X-shaped projectile and X-ray vision capability. Her abilities are not exceptional compared to veteran hunters, but she shows courage and effectiveness in combat when her emotions are engaged, ultimately proving her greatest strength is her capacity for love and sacrifice.