i'm a guardian
When everything's going slightly bizzare in the middle of the World That Never Was, and Riku appears with Way to the Dawn, a new keyblade, Sora's fighting Xigbar, Kairi escapes with Namine, and ends up going to help Sora, but is cornered by heartless in the process. At that time, she recieves a gift from Riku. That being, her keyblade.
This is, we know by now there's the keyblade for light (Sora's), dark (Mickey's), and dawn (Riku's). Logically, Kairi's should be twilight. But her keyblade really doesn't resemble twilight - it looks more like a mixture of her two homes, Radiant Garden and Destiny Islands. It has the sea, connected to the land and the sky. It's about this time Square tells us there are more keyblades than has really been let on. Anyone can wield a keyblade with a strong enough heart. Kairi's heart is strong and true - she did get through Betwixt and Between by herself, she stood up for herself, and she believes in her friends.
So, where did Riku get her keyblade? He either discovered it when he found his own, or, he'd had hold of it for a very long time and just not let Kairi have it. Maybe, if she had got it earlier, she would have gone to find Sora sooner, which he didn't want. Still, the keyblade excepts her when she takes it, bending to her will as she fights away the heartless. It accepts her.
Kairi fights in a similar style to Sora, but it's more of a cross between him and Riku. It probably comes from the all the years she spent watching the time of them fight each other. For this being the first time she fights heartless alone, she's pretty good at it. She's strong willed and keen, something that is definately needed for fighting the heartless. Why would she have recieved a keyblade in the first place? Well, maybe it's because she's a friend of the hero of light. She's a friend to him. Something like a 'guardian', perhaps. Just think of Final Fantasy X, and you'll catch my drift. I do wonder though if Kairi will keep her keyblade, since it was never mentioned afterwards. Perhaps we'll even learn more about it.