Kairi is a girl living in the Destiny Islands.
Kairi is appointed part of Tidus's team, the Crabs, in the children's game of Island War, alongside Wakka. Even though Selphie betrays the Seagulls to join the Crabs, Riku and Sora easily defeat the other four children.
After the battle, Kairi suggests a race between Riku and Sora. While resting, the three children talk about romance, although Kairi refuses to speak about her own love life.
That evening, Riku, Sora, and Kairi discuss the raft trip and other worlds. Riku credits Kairi's arrival during their youth for his idea to visit other worlds.
Kairi makes herself referee of Riku and Sora's race, unaware that they have agreed that the winner would share a paopu with her.
Kairi collects the last of the children's rations for the raft trip from Sora. She tells Sora how distant she thinks Riku has been, but Sora disagrees with her assessment.
Although planning to separate Sora from Kairi, Riku accidentally overhears their conversation. Kairi expresses concern at Riku's manner and suggests leaving him behind on the raft trip, before quickly clarifying that she is "kidding". After standing up for Riku, Sora goes to the secret place, where Riku follows him. Kairi witnesses their intimacy and interrupts, feeling hurt that she is suddenly a third wheel. Although she leaves quickly, Sora reassures her that she can come with them on the raft if she shows up.
The darkness assaults the Destiny Islands. Looking for Sora, Kairi goes to the secret place, but finds Riku instead. The Heartless begin to manifest. A mysterious man explains the Heartless's arrival, calling Kairi a princess and speaking of "two strong hearts, and a pure heart". The darkness envelops Kairi.