Kairi is the primary female supporting character of Kingdom Hearts, and remains as one of the most important people to Sora. For a large portion of the game, Kairi's heart disappeared, leaving her unconscious. Unbeknownst to Sora, her heart had been trapped inside his own since their departure from Destiny Islands. Her relationship with Sora seems to reach a much deeper level being each other's love interest. Perhaps the most important event of this relationship was at Hollow Bastion, when Sora turned into a heartless. Because of Kairi, he was able to transform back into a human, a rare ability that only a few people manage to accomplish. Her strong will compensates for her physical weakness, stemming from her caring nature. Towards the end, she and Sora decide to keep a promise: that when they meet again, Sora would return the Oathkeeper to Kairi. Sora refers to this keychain as his good luck charm, and would play a key element in his determination to reunite with Kairi someday.

After her departure with Sora at the end of Kingdom Hearts, she was left at her home on Destiny Islands while Riku and Sora were away. Kairi eventually forgets Sora’s name, but suddenly remembers it when her memories confront her of Sora’s existence along with Sora’s voice. When captured and held prisoner in The World That Never Was, she is rescued by Riku, and given her own keyblade to fight with. She finally reunited with Sora, and later runs to greet him upon their return to Destiny Islands at the end of the game.