In regards to how Roxas and Namine were born, I believe that the most commonly (almost unanimously, correct me if I’m wrong) held belief is as follows:
When Kairi and Sora’s hearts were released from Sora’s body, Sora’s body and soul were left behind as Roxas. Namine, on the other hand, was essentially created from nothing. It is a phenomenon AtW had earlier hypothesized in the Secret Ansem Reports when he believed that Namine came into existence when Kairi lost her heart (rather than when Sora lost Kairi’s heart).
I myself believed this unquestioningly for years.
But not too long ago, when I had been scrutinizing Another Report and the Secret Ansem Reports (for unrelated reasons), I by chance, came to another conclusion.
It is the exact opposite of what people have assumed. Namine, not Roxas, was born through Sora’s body and soul. Roxas, on the other hand, exists through unknown means.
I know there’s going to be an overwhelming amount of opposition to this idea. But hear me out. Give me the benefit of the doubt to explain this. It is NOT my theory, it’s what I have concluded that Nomura is saying. I’m just the messenger boy.
The bulk of what I’m about to say involves grammar. If you’re not familiar with the terminology or the concepts, rest assured that I am not leading you astray. I consider myself a grammar buff and, trust me, I know what I’m talking about. If you have any doubts, by all means bring them up and I’d be glad to give you a little tutoring. But don’t disregard it simply because you don’t understand.
You might scoff at how much I’m analyzing the grammar of a meager two or three paragraphs. But I do so in your best interest. If the vague, seemingly ambiguous structure in the sentences of Another Report/Secret Ansem Reports can fool me (as well as the entire KH community) this easily, then it deserves some serious examination.
Let’s get to work then.
"Essentially when a strong hearted person has their heart stolen, they change into a heartless, and on rare occasions their body changes into a Nobody. But in the case for these two, for Kairi's heart to be hidden within Sora, it took a special shape."
So let's take a look at that second sentence:
"But in the case for these two, for Kairi's heart to be hidden within Sora, it took a special shape."
The word “it” is behind most of the trouble here. What is the antecedent of "it”? Well, many assume it to be Kairi's heart.
But that is grammatically incorrect. "Kairi's heart" is in the prepositional phrase whereas "it" is in the independent clause. Because the prepositional phrase cuts in between the independent clause before the word "it," many people have that misconception that it=Kairi's heart (since “heart” is the closest noun that would be considered an “it.”). Plus, Kairi’s heart taking a special shape doesn’t really make sense in reference to special Nobodies, does it?
If we cut out the prepositional phrase to see the independent clause as a whole (which is fine), it reads, "But in the case for these two, it took a special shape." By reading the sentence as such, you can tell that the antecedent of "it" is in the previous sentence. So what noun could it be talking about?
The answer is "body." That is, the body ordinarily changes into a Nobody, “BUT” (used as a conjunction in the next sentence to refer to an exception) it (Sora’s “body”) took a special shape. When we read in it that context, the two sentences make sense grammatically as such:
Essentially when a strong hearted person has their heart stolen, they change into a heartless, and on rare occasions their body changes into a Nobody. But in the case of Sora and Kairi, Sora's body took a special shape because Kairi's heart had been hidden within it.
This is irrefutable. You cannot deny that Sora’s body is what took a special shape.
Now it’s a matter of deciphering semantics.
So what does he mean by "special shape" here then? How does Sora’s body take a special shape?
Let’s think. He says that, ordinarily, the body changes into a Nobody, which would be considered the original person’s Nobody. BUT Sora’s didn’t, it took a special shape. Sora’s body taking on the shape of Roxas, his Nobody, would be normal. Catching on? His vessel did not take the shape of his own Nobody, but as Kairi’s Nobody. Namine.
And you can note that he’s actually saying the same thing that Ansem the Wise said:
"I believe that Naminé was born as a special type of Nobody when Sora attacked himself with the Keyblade, causing his and Kairi's hearts to leave their bodies simultaneously.
Naminé emerged as Kairi's Nobody… but the body and soul necessary to exist as a Nobody belonged to Sora.
[...]
Naminé is Kairi's Nobody, but came into being via Sora's body and soul."
When AtW says, “but the body and soul necessary to exist as a Nobody belonged to Sora,” it does not mean Namine didn’t /use/ Sora’s vessel, but rather that DESPITE being Kairi’s Nobody, she was made from Sora’s body and soul.
And as for Namine coming into being “/via/ Sora’s body and soul.” It’s exactly what it says. ANY Nobody comes into being VIA the body and soul. While we usually consider the Nobody to be comprised of the same body and soul as their original, they technically aren’t. If they were, they would be existent (since a body and soul are definably existent) and would look exactly like their Somebodies (think of Kairi’s empty vessel when she loses her heart). Thus, they are born FROM (via) the left over body and soul.
