Kairi is a friend of Sora and Riku, and one of the seven "Princesses of Heart". She is an original character created by Tetsuya Nomura for the Kingdom Hearts series.[1] Nomura has stated that Kairi's name can be seen as "sea";[17] the Japanese on'yomi pronunciation of sea is kai (海, kai?).[16] Kairi is featured in both Kingdom Hearts and Kingdom Hearts II. She is voiced by Hayden Panettiere in the English version and Risa Uchida in the Japanese version.

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.[23] At the beginning of the series, she is 14 years old.[19] 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 which she uses to fight off the Heartless that Saïx summons. Kairi and Riku are reunited with Sora, and along with Donald, Goofy and King Mickey, the group face and defeat Xemnas. Kairi also bonds with Namine in the process and she returns to Destiny Islands, Sora and Riku following shortly after.

Naminé is an original character created for the Kingdom Hearts series. She has the ability to change memories of those in relation to Sora by rearranging them and creating new links between them. Naminé is voiced by Brittany Snow in the English version of Kingdom Hearts II and Iku Nakahara in the Japanese version. Naminé first appeared in Kingdom Hearts: Chain of Memories. Under orders from the Organization, she altered Sora's memories to remember her as a close friend from Destiny Islands, and eventually replacing Kairi. She almost completed her task until Axel allows her to leave his sight. Naminé then manages to reveal herself and the truth to Sora. Once Marluxia is defeated, Naminé has Sora, Donald, and Goofy step into devices that will put them to sleep for a year so she can rearrange the chains of their memories, but break the ones that she created.[21] As a result, Sora's memories of Naminé will be erased. Naminé also helps save Riku in Reverse/Rebirth from Zexion by taking on Kairi's appearance to convince Riku not to refuse the darkness within his heart while drowning in light. When they meet in person, she offers to lock away the darkness of his heart so that it can never hurt him again, but he refuses the offer, opting to battle the darkness. Afterwards she contacted DiZ to help protect Sora, Donald and Goofy.[8]

In Kingdom Hearts II, Naminé is revealed to be a "special Nobody" born from Kairi's heart when it left Sora's body. As Kairi had no darkness in her heart, her heart hid within Sora's body instead of becoming a Heartless when Destiny Islands was consumed by darkness. When Sora impaled himself with the dark Keyblade, his body and soul were used to create not only Roxas, but Naminé as well. As a result of this, she has none of Kairi's memories. Naminé's special birth in conjunction with the fact that Kairi's heart had been in direct contact with Sora's is why, DiZ states, that Naminé's special powers could only affect the memories of Sora and those whose hearts are connected to his, which was why she was used by Organization XIII. Despite DiZ's orders, Naminé visits Roxas in Twilight Town. She appears to him several times, often before he is attacked by Nobodies, acting somewhat as a guide to him. Riku was instructed by DiZ to destroy Naminé since he has no more need of her. However, as seen in Kingdom Hearts II Final Mix, Riku decided to let her go with Axel to repay the favor to her for Castle Oblivion. She is not seen again until near the end of the game where she frees Kairi and Pluto from imprisonment and they attempt to escape from the Organization's stronghold. Naminé gives up her own existence as an independent entity and fuses with Kairi to complete her.