here my theory of Kairi: she was still on Destiny Islands during KH:CoM. Sora voice was inside of her, but maybe at the beginning of KH2, Sora voice left her. if you see in Deep Dive on the "10" of the countdown, the quotes said, "His voice, It left me" & "This time...I'll fight". So Kairi learns that Sora may be in danger and she must help him. I think she made a raft, sail off, and landed on a island where the Red-Haired Girl is which may explain that Kairi is the Red-Haired Girl. I really believe Namine is Kairi shadow and the girl on the beach in ASAS is NOT Kairi, but may be a new character on another island. And I think Kairi weapon will be a bow-arm which is the weapon that Rinoa from Final Fantasy VIII used.
EDIT: it would be nice if someone would comment on my theory.