Don't ever forget. Where ever you go, I'm always with you.
Sora, are you really— No. It can't be! I won’t let him go!
~ after Sora disappears.
I'm tired of waiting, so don't tell me to stay behind! From now on, wherever one of us goes, the other follows, got it?
~ before the final battle.
Thinking of you, wherever you are. We pray for our sorrows to end, and hope that our hearts will blend. Now I will step forward to realize this wish. And who knows: starting a new journey may not be so hard, or maybe it has already begun. There are many worlds, but they share the same sky — one sky, one destiny.
~ while Sora reads her letter.
Kairi is the tritagonist of the Kingdom Hearts video game series.
She's a Princess of Heart and was from Radiant Garden (Hollow Bastion) and currently resides on Destiny Islands. Her grandmother's status is unknown.
She's the love interest of Sora.
Kingdom Hearts Birth by Sleep
Aqua and Mickey save Kairi from being attacked by Unversed in Radiant Garden, and she gives the former flowers as a sign of her gratitude. Aqua is also able to sense the light inside Kairi. After the Unversed are defeated, Aqua casts a spell on her that will allow her light to lead her to someone to keep her safe when she's in trouble. She then thanks Aqua and leaves with her grandmother, who tells her the story of light and the darkness.
A year after the events of Birth by Sleep, Radiant Garden was covered in darkness, and all of its inhabitants were whisked away to different locations across the universe, with Kairi ending up on Destiny Islands due to the spell Aqua cast on her a year earlier.
Sometime after this, Sora and Riku talked about "the new girl at the mayor's house" in their Secret Place on the island, referring to Kairi.
At the beginning of the game, she and her friends are making a raft to go visit other worlds, and her body gets whisked away to Hollow Bastion when the Heartless attack the island, with her heart taking residence inside Sora. Due to this, Sora sees multiple visions of Kairi throughout the game, the first time being when he wakes up in a motel room after his fight with Leon, when he sees Yuffie as her until she tells him her name, a second time when he visits Merlin's study in Traverse Town, when she tells him that the study reminds her of their secret place on the islands, the third time in Deep Jungle, where Sora briefly sees her behind Tarzan. He eventually finds her in Neverland, where Riku has her body at Hook's Ship. Before Sora, Donald and Goofy set off for Hollow Bastion, Kairi tells Sora from within him that she believes in him, and then shows him her memory of her grandma telling her a story about the light when she was younger. When Sora sacrifices his heart in Hollow Bastion, she is revived. She appears at the end and Sora promises to come back to her soon which she know he will.
Kingdom Hearts: Chain of Memories
She was replaced by Naminé with her magic and remembered at the end of Sora's story. In Riku's story, she was in Namine's form and not forgotten by Riku.
Kingdom Hearts 358/2 Days
Kairi is only seen in flashbacks and mentioned by Riku and Xion.
Kingdom Hearts II
She comes into contact with Roxas in minds which allowed Sora to talk to her. She was kidnapped by Axel and Saïx at different times. Later, she reunites with her friends and Nobody Naminé. At the end, she went back home to Destiny Islands and reunites with her friends there.
Kingdom Hearts 3D: Dream Drop Distance
When Young Xehanort makes Sora fall into a second layer of sleep in The World That Never Was, he dreams about various people he knows, one of them being Kairi, who walks away from Sora with Riku. She then turns into Aqua while Riku's dream self turns into Terra, and Sora himself turns into Ventus. They then walk away from Sora and turn back into Riku and Kairi.
Later, when Xigbar brings the strength of Sora's heart into question, he pauses for a moment before retaliating by saying that although the Keyblade didn't choose him, he is proud to be connected to those it did, at which point all of the friends in Sora's heart, including Kairi, appear behind him for a moment.
Knowing that Xehanort is forming a new Organization XIII, Yen Sid tells later Riku to to get Kairi so he can train her to use the Keyblade.
Kingdom Hearts 0.2 Birth By Sleep -A fragmentary passage-
When Kairi arrives at Yen Sid's Tower, he tells her about the new Organization XIII and the fates that befell Terra, Ventus and Aqua. Mickey reveals to her and Riku that Aqua sacrificed herself to protect Riku from the Heartless when they were closing the Door to Darkness, staying behind in the Realm of Darkness. After Mickey and Riku are given the task of rescuing Aqua, Kairi asks what her task will be, and is told that she and Lea, Axel's original self, will be training together under Merlin to use the Keyblade. This makes Kairi shocked because Axel kidnapped her before, but Mickey and Riku assure her that he is now on their side, as he saved Sora from becoming one of Xehanort's thirteen vessels. She then leaves for her training.
Between Kingdom Hearts 0.2 Birth By Sleep -A fragmentary passage- and Kingdom Hearts III
As Kairi trains with Lea, she is still somewhat wary of him because of his kidnapping of her as Axel, but she gets used to him over the course of their training and form an acquaintance with each other, as she can't bring herself to hate him after finding out that he also wants to save a friend. Kairi sometimes catches Lea staring at her face, and when she asks him what's wrong, he says that he doesn't know, but that he feels like there's something he should be remembering.