Kairi

Kairi (カイリ) is a 14-year-old girl and is one of the three main characters appearing in Kingdom Hearts, Kingdom Hearts II and Kingdom Hearts: Chain of Memories. She is the love interest of Sora and Riku, who compete for her in the first part of the series, although it would seem she had "chosen" Sora as of the end of Kingdom Hearts II.

Kingdom Hearts
In the original game, Kairi is attacked by Heartless, which leaves her as an empty shell. When Sora and Riku are taken from the island, they are seperated from Kairi, and each other. Riku later finds Kairi, and uses the power of darkness to restore her (which eventually leads to Riku being possessed by Ansem). Sora also tries to find her, to no avail, until he (along with Donald and Goofy) is captured by a pirate ship. On the ship, he finds Riku, who is taking care of Kairi, and trying to heal her. Riku takes her to Hollow Bastion, and it is revealed that she is one of the Princesses of Heart. Sora and Riku fight, and Sora wins, leaving him control of Kairi's empty body. By using Riku's Keyblade, and by sacrificing his own heart, Sora brings Kairi back to life, and Sora is turned into a Heartless. Kairi, Donald, and Goofy later find Sora in his Heartless form. Although Donald and Goofy begin to attack him, Kairi realizes that the Shadow is indeed Sora, and by showing him her love for him (by protecting him from other Heartless), Sora is changed back to his human self. In the conclusion of the game, Kairi and Sora are seen after Sora locks Kingdom Hearts. Kairi is sent back to the Destiny Islands, while Sora is transported to a different world.

Kingdom Hearts: Chain of Memories
Although Kairi did not appear in Kingdom Hearts: Chain of Memories, she did play a very important role. In Sora's story Naminé tried to take Kairi's place in Sora's heart. However, she said that she could never replace Kairi and that she was only Kairi's shadow. When Sora was about to go to sleep she told him that if he remembered Kairi's name, his memories would restore faster. In Riku's story, Naminé took Kairi's form to help Riku control the light and darkness in his heart. In the credits for Kingdom Hearts: re-Chain of Memories, Kairi is shown waiting for Sora and Riku's return.

Kingdom Hearts II
Kairi, now 15 years old, lives her life normally on Destiny Islands. She misses Sora, but because he was inside of the flower pod, which he was in to restore his memory, she could not remember his name. After the conclusion of Roxas's story, she remembers his name to be Sora. One day, Axel comes to the Destiny Islands and tells Kairi to come with him. She refuses until Axel summons a group of Nobodies. She then runs off into a near-by portal with King Mickey's dog, Pluto, into Twilight Town and meets Hayner, Pence and Olette. Eventually, she is found by Axel who takes her to The World That Never Was as prisoner and is eventually freed by her Nobody, Naminé. The two attempt to escape but are stopped by Saïx and some Nobodies who offer to take Kairi to Sora. Once she refuses, Saïx is about to send the Nobodies to attack the two girls only to be saved by Riku, whose appearance has changed to that of Xehanort's Heartless. Riku leads her to Sora and is attacked by a group of Heartless. He hands Kairi a Keyblade, and they successfully fend off the remaining Heartless. She is then reunited with Sora and shows him that the look-alike of Xehanort's Heartless is really Riku. After Sora and Riku lock Kingdom Hearts, she is sent back to the Destiny Islands, but is once again separated from her friends. After they defeat the final boss, they also return to the Islands, and the game ends.

Personality and Abilities
Kairi is a caring and kind-hearted girl, but despite her seemingly delicate appearance, she can be extremely determined and courageous when she needs to. She also can act rather spunky and tomboyish when she hangs out with Riku and Sora. She makes new friends easily, and quickly befriends Hayner, Pence, and Olette after fleeing from Axel. She is also fiercely protective of her friends, even to the point where she is willing to risk her life to protect them, as seen when she throws herself into danger twice to try to help Sora. While the mystery of her origin would bother others, she takes it in stride, and enjoys her new life on Destiny Islands.

Kairi's abilities in combat however, are fairly inadequate. Kairi unfortunately isn't able to do much when she herself is in danger, and often depends on Sora or Riku to come to her aid. She does, however, have the rare ability of being able to wield a Keyblade. This is curious as her Keyblade has a chain, which means that it, unlike Artificial, is real. Besides this ability, she also has the powers of the Princesses of Heart, and her heart inherently has no darkness whatsoever within it, which lead to the unique birth of Naminé. Her being a Princess of Heart may also be the reason why she has the ability to weild her keyblade.

Trivia

 * Part of her name ("Kai") is the Japanese word for 'sea' and goes with the overall "naming theme" of Kingdom Hearts main characters (Sora means sky and Riku means shore or land).
 * Kairi is voiced by Hayden Panettiere in the English version, and by Risa Uchida in the Japanese version.