Keiko Onuki (known as Keiko Inoue in the manga version) is a character in the novel, film and manga Battle Royale. She was played by Minami Hinase in the film version.

In the novel, Keiko Onuki was in Shogo Kawada's 9th grade class in Kobe Second District Junior High School in Kobe, Hyōgo Prefecture. In the film, Keiko is with Shogo in Class 3-C in Kobe Number 5 Junior High School in Kobe.

In all versions of Battle Royale, Keiko and Shogo were forced to participate in a Battle Royale.

In the novel, it is stated that Keiko dies before meeting Kawada and that he stayed at the location of her death for quite some time (he claims it was because "she died a horrible death").

In the manga version, she is the target of a desperate search by Shogo. When she sees him killing his first victim, she runs away from him. In his search, Shogo ultimately went on a killing spree comparable to Kiriyama's in the later Program. When he eventually found her, she was being held hostage by an obsessed stalker who had been watching their relationship in anger and jealousy for some time. After tricking and killing him, the two are left to figure out what to do next. Shogo was intent on spending his last moments with her, before killing himself so that she could win the game and be free. Soon after he explained his idea, she dove for a gun and aimed it in his direction. Instinctively he dodged and reflexively shot her. Only to discover, with the sight of two blips on the tracker, and the announcement from behind, that there was another student still living. Keiko had been aiming at her, and he had shot her dead without even realizing that she was trying to save his life.

In the film version, in the final hour of the game, Shogo and Keiko, who both appear to be injured, walk to a hill where a massive battle occurred, with dead bodies lying around them everywhere. As the final minutes approach, the collars start flashing red and start beeping. Knowing that their death is approaching, the two embrace and prepare themselves, but before the collars detonate, Keiko shoots Shogo. Shogo rolls down a hill, and shoots Keiko, mortally wounding her. She smiles, says "Thank you...", and dies.

The collars stop beeping, and Shogo wins. This serves as his motivation in the film — he wants to know why she gave up her life for him, something he eventually discovers when he does the same thing in the second game.

In all versions, Keiko is a powerful driving force for Shogo, but the mystery of 'Keiko's last smile' is present only in the movie.

The first character of her name, "Kei," means "joy." The "Yoshi" in Yoshitoki Kuninobu is made up of the same character.