The more I think about it, the more I don't get Namine. She looks like Kairi, but with blonde hair, so the blonde hair could imply some kind of connection to Ventus. Also, from what we know she was born in Castle Oblivion: the place where Ven's body and soul are peacefully sleeping until Sora can find Aqua who can 'solve' the mystery of CO so that he can give Ventus his heart back.
So we've got Roxas, Sora's Nobody in every sense except that he looks like Ven and started with none of Sora's memories, only to start getting them after the creation of Xion, and contained a fractured piece of Ventus' heart inside of him. We can write off Xion's involvement because per this discussion she's not important. When Namine was created Sora still had all of his memories of Kairi and she was even RIGHT THERE when he turned himself into a Heartless to give her heart back to her...
Would it be possible that Namine was also born with a fractured piece of Ventus' heart inside of her, and Sora's last moment when Kairi grabbed his vanishing body be the catalyst that caused her to look like the spiky-haired brunet's love interest? Namine having a piece of Ven's heart would explain how she was able to feel emotions, because I'm pretty sure that she can feel just like Roxas could, but there's still the mystery of where her body and soul came from if Roxas had Sora's, unless the piece of Ven's heart provided her with her own.
However, if the above is true, then that means that Kairi now also has a piece of Ven's heart inside of her after merging with Namine at the end of KH2, so she will be needed when the time comes for Sora to save Ventus.
Congratulations Nomura, you have successfully created the biggest mind-screw game of all time!