~The BHK is a different side of Sora, and Namine is a different side of Kairi. Namine and the BHK could very well be siblings, because of their blonde hair. This does not imply that Sora and Kairi are related, because that would totally ruin the story. <--Actually, there's another reason I thought of, but I can't think up of the words right now.

Anyway, if you've noticed, the BHK's facial features(of what we have seen of them) look similar to Sora's, and Namine's looks similar to Kairi's. Okay, now that we've gotten a good enough grasp of the situation, let's go on.

We all know that Sora lost his memories and he has to go into Castle Oblivion and reclaim them. Apparently, he also has to rescue Namine. But maybe, his real goal(though he doesn't know it) is to rescue his memories of Kairi! Namine, who I believe to be a different side of Kairi, holds the memories of Kairi inside her, and when Sora rescues her, his memories of Kairi will come back to him. Because Namine and Kairi are basically the same person as a whole, she gives him the paopu fruit to prove that she is one and the same. Whew.

~And how Riku ties into all this I have yet to think of. Of what I can think of currently, does Riku's fate lie in Namine's hands? Because according to a translation, apparently, it does. So then, the theory would be, depending on if Sora rescues Namine, is where Riku's fate lies.

*These Theories call for more thinking, so I'll have to post later. *