Keiko is a pivotal character from the Battle Royale narrative, appearing across the novel, film, and manga adaptations, though her story and fate differ significantly in each version. Her primary significance lies in her relationship with Shogo Kawada, a key figure in the main storyline, as her actions and death serve as a powerful driving force for his motivations. In the novel, Keiko is stated to have died before she could reunite with Shogo, and he remained at the site of her death for a considerable time, haunted by the horrific nature of her passing. In the manga adaptation, where she is sometimes known as Keiko Inoue, she and Shogo had been dating. Their relationship was marked by philosophical disagreements, particularly about the role of emotion versus logic, with Keiko firmly believing that the heart and emotions could serve as a valid form of reason and help define a person's character. This argument had a lasting impact on Shogo, influencing his actions long after her death.

When forced to participate in the Program, Keiko becomes the target of a desperate and violent search by Shogo. After witnessing him commit his first killing, she becomes frightened and flees from him. During his search, Shogo embarks on a killing spree. When they finally reunite, Keiko is being held hostage by a jealous stalker from their school. After Shogo kills the captor and explains his plan to spend his final moments with her before killing himself so she can win, Keiko dives for a gun and points it in his direction. Shogo, acting on instinct, dodges and shoots her, only to realize that she was actually aiming at another student who had been behind him, trying to save his life. Her death in this version is a tragic misunderstanding.

In the film adaptation, Keiko and Shogo survive together until the final hour of the game. Both appear injured and walk to a hill surrounded by the bodies of their fallen classmates. As their explosive collars begin to beep and flash red, signaling their imminent detonation, the two embrace. At the last moment, Keiko shoots Shogo, who then reflexively shoots back, mortally wounding her. As she dies, she smiles at him and says Thank you. Her death makes Shogo the winner of that Program. This act of sacrifice and her final smile become the central mystery and motivation for Shogo in the film, driving his desire to understand why she gave up her life for him. Her personality, as glimpsed through these interactions, suggests a deeply emotional and selfless individual, capable of great love and ultimate sacrifice. Her primary role in all versions is as a tragic figure whose death profoundly shapes Shogo Kawada. Her key relationship is exclusively with Shogo, as her boyfriend and the person she ultimately tries to protect. Her development is seen only in flashback or as an influence on Shogo, evolving from a girl with strong convictions about the heart to a person who makes a final, desperate choice to save the one she loves. No notable combat or survival abilities are depicted for her across the different versions.