Say this. Nobody knows what Namine is. I think that maybe, just maybe, that she is a Lingering Sentiment. I know how impossible this may seem but... I guess that could explain why she is and was so... different.
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Sora's battle with Terra's Lingering Sentiment took place in his own time. Terra's LS was still brimming with hate - maintained by it - more than a decade after the events of Birth by Sleep.All the Sentiments weren't around for very long.
Terra's Lingering Sentiment= about 5 min TOPS
We haven't exactly been able to gauge the Linger Spirits' intelligence, but I think it's safe to say that they are sentient and self aware.But Namine demonstrates a range of characteristics - such as intelligence, memory and emotion - that all other Lingering Sentiments thus far lack, not to mention her birth was different to the creation of all understood Lingering Sentiments.
I think that this actually sounds pretty good. Agreed majorly with the last part though.I still like my pet theory that she was housing Ven's vessel though. Will certainly tie in with where she was born. Ven's Heart escaped his own vessel into Sora so he had no point or strength to return, or even leave Sora's vessel. Namine though would've reached Ven's vessel through Sora and Ven's bond. Hence, while having Kairi's face, she has Ven's colors. Then the vessel could've faded away and the 'Self' would've returned to Kairi between when Kairi and Namine met Riku, and Riku and Kairi meeting Sora. There was enough time for them to pull something like this off. Hell, I dare say part of why we never saw what happened there was so Nomura could bide his time and come up with something trollish enough to shove in there.
She might have been created using that same method, but with Sora.A physical manifestation of what remained of Vanitas's powerful thoughts and feelings.