You're probably going to think i'm an ''idioit'', but whatever, these are just ideas i'm not saying that they are true.
Okay, First ''Theory'': Sora is somewhat a yandere
In the kh2 novel he says this(Or something very similar; Japanese can have different meanings):
“I don’t have time to play around with you!” Sora yelled, leaping at Xigbar. “I’m going to see Kairi!”
And then: “I’m coming, Kairi!” He sounds kinda like yuno
. Sora has anti-form/problems with his darkness, so it wouldn't be to far-fetched. Also, It wouldn't fit badly with sora's character as a yanderes act sweet and kind then violent.
. Sora has killed/tried to kill/gotten angry at orgy members when they threaten etc Kairi/Namine
. Acts obssesive over Kairi/Namine (Though this can be taken as just caring alot)
[i reference the manga sometimes so events might not be in a cutscene strictly speaking but still second-tier canon]
Second theory: Sora and/or kairi might faint if they enter castle oblivion
Ventus(And Aqua, though i doubt that would matter much), Roxas, Xion and Namine have all been there ; Roxas/Xion Nearly fainted simply going inside
People retain memories of their time as a nobody/ Kairi could possibly see Namine's (Xion's, too, IF you count xion as a round-a-bout part of kairi) memories?
I admit, this one is more of a wild guess; Kairi could just say something like, ''I feel like i've been here before'' (Well, if she goes with them and they find aqua and C.O that is.)
This one isn't really a theory, but anyway, third theory: Kairi's fighting style/is that of a two-handed sword
Because kairi weilds her keyblade with two hands and the length of the blade seems long, i can see kairi fighting with sweeping attacks (Which goes well with her name being ocean themed).