Personality;
Sweet, nice girl, with unyielding will. Despite her general kindness and extremely pure heart, she isn't afraid to stand up and fight when her friends need that. She also shows her laid-back parts of personality whenever she slacks off with Sora, and in kind of rebellious way do everything to stay out of doing what she have to do. Even if everything she was put through her story she had to grow up, she still sometimes shows that parts of her personality.
After the events that changed her life entirely and forced her to be away from her best friends for whole year, she became a lot more sincere, and gets sad a little more easier than before. Kairi makes new friends really easily, as she's very out-going person, open to basically everyone (except people from Organization XIII, maybe).
Strengths/Abilities;
Her own heart, possibly, being one of the most purest of them all. Given the flowery & totally girly Keyblade thanks to Riku, she's also able to fight with it - although simply protecting herself and her friends a bit, as she's both not that athletic nor a fighter, she simply learnt a bit from watching her best friends training.
Weaknesses;
She keeps falling into trouble, and Bad Guys seem to enjoy kidnapping her for their own purposes, and to their content, Kairi can't run away from them. And as stated before, she doesn't have extraodrinary powers, so even if her mind keeps forever strong, the body doesn't follow sometimes.
She can also be incredibly stubborn and clueless about what's safe and what isn't; she probably gave Riku a heart attack when she jumped from quite big height only to give Sora a hug.
Background;
Kairi was born in a world called Radiant Garden. The only thing she remembers of that time is, strangely, talking with her Grandmother in large library, asking her for some stories. Even if she's not quite aware of this, she was chosen by person named Xehanort to become one of Seven Princesses of Heart, and then, send to other world to find future Keyblade wielder.
And so, on the night when stars were falling down from the skies, little girl was found on one of the Destiny Islands' shores. Except for her name, she couldn't remember - or wasn't willing to tell - anything from her past. It was then when Kairi met Riku and Sora - two boys, her two best friends, their destinies forever binded with her. She spend few years on that island, her best time in the whole world, being with their friends and doing nothing. But her sudden appearance on the islands made Riku think about crossing the sea to visit other worlds; and while that scared Kairi because she was afraid that her happiness will be lost by that, she agreed to go with him and Sora when they'd built a raft. And so, as Riku promised, they managed to build one.
However, the night after, the whole world went mad. Kairi's heart was lost and she disappeared somewhere, Sora got driven to another world, and Riku surrounded himself to darkness.
It was also the moment when Naminé , her Nobody, was born.
Being an empty shell, she wasn't aware what was happening all the time when Riku and Sora were struggling so much. When she woke up after Sora pierced his heart to free her own, and realized that her beloved friend became a Heartless because of this, Kairi without thinking twice held him close to her heart and saved him by returning Sora to his original form. After that, she was sent by Sora to Traverse Town, where she stayed for a while with Yuffie, Aerith and Leon until they return to Hollow Bastion. Kairi gave Sora her good-luck charm, a little star, with promise: Bring that back to me. And don't ever forget. Where ever you will go, I'm always with you. And so she had to tell her friend goodbye once again, as they got seperated by much stronger forces they could ever imagine.
When she returned, alone, to Destiny Islands', she also returned to normal life. She still visited the island, had fun with Selphie, Wakka or Tidus, went to school... But something was wrong. She could remember Riku, her best friend that disappeared year ago; but inside her heart, Kairi felt there's a place for another important person. She couldn't even remember his name, she was thinking about him endlessly, but nothing came. When she told Selphie about him, she even said that there wasn't no one like him, and Kairi probably just made him up. But the Princess knew it wasn't the case. Kairi even wrote a letter, straight from her heart, addressed to that boy. But she promised herself that she won't send it; not until she remembers his name.
Until the most bizarre meeting in her mind; the moment when she spoke with a boy named Roxas, and heard that boy's voice in her head. It was then when she remembered - starts with an S, right, Sora?. And she was finally able to send this letter to him, and so she threw her message in a bottle into the ocean.
Some time later, she was standing on the beach, thinking that maybe waiting isn't good enough, that maybe something else is needed. And in that moment she met a member of Organization XIII for the first time - Axel. He kidnapped her, saying that he will make it so she'll meet Sora. Seeing Nobodies along side him, she knew right away that he lied, and tried to run away from him - then, she saw a dog, heard a whistle and followed them both into the portal of darkness (probably sensing somehow that it was made by Riku). That was a moment when Kairi decided to go on her own journey to find Sora and Riku, and ended up in Twilight Town, where she met Hayner, Pence and Olette, and quickly befriended them. Once again however, she was kidnapped by Axel, run away again, and then the one who took care of her was a man named Saïx, who threw her in the cell in The World That Never Was for good (or so he thought, silly man).
This time, her own Nobody, Naminé, rescued her. When the two of them were trying to get out, Saïx showed up to stop them once again (do this man never learns?) - only to wait for a black-hooded man to come and this time stop Saïx himself. Naminé tells Kairi then that the man is Riku, and she quickly runs to him, and believes in him so strongly that she wasn't even taken aback by his Ansem's appearance. Then, Kairi saw Sora fighting bunch of Heartless, took Riku along and decided to fight alongside both of them.
And so, three friends were reunited again. Kairi helped Riku to return to his old self; after Sora and him defeated Xemnas, all of them returned to Destiny Islands to live happily ever after...
Or not.