Kairi is 16 years old, and lives on Destiny Islands with her friends Sora and Riku. She is happy-go-lucky, but dreams of finding a way off of the Islands to visit other worlds with her friends. Her past is mysterious; she just appeared on the Islands one day. Sora and Riku are both very protective of her, although she seems to be closer friends with Sora; they used to spend time together in the "secret place" when they were small, drawing pictures on the walls. When the Heartless invade the Islands, however, the three friends are swept apart. So begins Sora's quest to find Kairi..
When Sora finally finds Kairi on the pirate ship in Neverland, she is unconcious because, as Riku tells Sora, she has lost her heart. Riku takes Kairi away with him to Hollow Bastion, where he meets up with Maleficent. Maleficent has been controlling the Heartless; she has been searching the worlds for the seven "Princesses of Heart," who hold the key to unlocking the huge keyhole in Hollow Bastion. As it turns out, Kairi is one of these princesses, but she cannot aid in opening the keyhole because her heart is missing.
When Sora comes to rescue Kairi, he saves Kairi's heart when he realizes that, because they are such close friends, part of her heart resides within him. So he gives himself up to the darkness to restore Kairi. In doing so, he becomes a Heartless. Kairi saves him, however, by holding him close to her and protecting him from the other Heartless that are attacking them.
At this point, Sora returns Kairi to Traverse Town, where she stays for a while, apparently under the care of Yuffie, Leon, and Aerith until they return to Hollow Bastion. Kairi gives Sora her good-luck charm, and a keychain for the Keyblade, called the Oathkeeper. She also helps Sora find the Navi-Gummi hidden in the secret waterway which points the gummi ship in the direction of the End of the World.
When the Heartless are finally subdued and Ansem is defeated, Kairi appears where Sora is; however, they are being pulled apart by an unseen force, which is sending each person back to the world where he or she belongs. Kairi is being taken back to Destiny Islands, but Sora must remain where he is so he can close to door to Kingdom Hearts. Sora tells Kairi he is always with her before she disappears back to Destiny Islands. Sora then closes the door to Kingdom Hearts, locking King Mickey and Riku inside.
In the final FMV sequence, we see Kairi in the "secret place" on Destiny Islands, remembering her days with Sora drawing on the walls. A tear slips down her cheek; Riku is trapped in Kingdom Hearts, and Sora has not yet returned. Obviously there is still business for him elsewhere..