I doubt that Kiari will be a playable character........One of the important-game- producer-people said that there would only be two playabe characters, and I'm guessing that the other character will the BHK. Why else would he not only get two Keyblades (How come he gets two anyway? But he's too short to be the Unknown in the secret movies. What, is it some kind of fad?) but also his own Awakening sequence as well? And he certainly seems to have a rather large place in the plot. Anyway, It's also doubtful that Riku, or anyone, can teleport between worlds. Becuase then the Heartless wouldn't have ships. Only Ansem seemed capable of that, but then again, Ansem's just really weird.
But remember one of the quotes from Deep Dive?
This time....I'll fight.
Maybe that's Kiari. I don't really think she'll get a Keyblade-why wouldn't she have gotten one before then?-but maybe she'll do her whole special Princess-of-Heart thing. Magic or something. Whatever happens, my bet is that she won't stay on Destiny Islands. Because I'm positive that what we saw in ASAS with Kiari was a wall breaking down. But what caused it? In ASAS, it almost looked like Riku was causing it, or at least he knew about it, because he was looking at the sky. But that didn't happen in DD. So confusing!