Kairi is a friend of Sora and Riku, and one of the seven "Princesses of Heart". She is originally a resident of Hollow Bastion, but she was sent to the Destiny Islands by Xehanort a few years before the events of Kingdom Hearts. At the beginning of the series, she is 14 years old. During the events of the first game, her heart was separated from her body which led to Sora and Riku both searching for a way to revive her. Kairi's heart was discovered to be in Sora's body, at which point he gives up his heart in order to restore hers, transforming himself into a Heartless. Soon after, Kairi finds Sora's Heartless and restores him back to human form. After Ansem's defeat, Kairi is left behind on the reforming Destiny Islands and separated from Sora at the end of Kingdom Hearts. Sora promises to return to her with Riku, and Kairi waits on the island for their return.
In Kingdom Hearts II, Kairi and all of Sora's friends slowly begin to remember Sora. After Sora's memories had been restored by Naminé, Kairi sends the letter drifting to the sea in hopes of that it will find its way to him. She is later confronted by Axel, who hopes to use Kairi to lure Sora to him. With the help of Pluto, Kairi evades him and heads to Twilight Town, but is captured by Axel soon after. However, Saïx takes Kairi from Axel and imprisons her and Pluto in a jail cell in The World That Never Was. She escapes her cell with the help of Naminé and they are saved from Saïx by Riku, in the guise of Xehanort's Heartless. Riku gives her a keyblade in which she uses to fight off the Heartless that Saïx summons.