Okay folks, I'm gonna need your help with this one. Yeah, we're going to try and figure out exactly how Xion came to be; it's gonna take a group effort. I'm gonna throw out some ideas, you guys toss some of your own, and eventually, we'll figure out something, k? Alright! ^^
Idea #1
So, let's see here. First off, I'm pretty sure we're all aware that Namine and Roxas are very unusual nobodies, no?
Namine ~ Entity of Kairi's heart, created with Sora's body and soul.
Roxas ~ Entity of Sora's heart, created with Sora's body and soul.
Strange right? Anyway, did you also know that Kairi techincally released her heart twice? First being how her heart magically ended up in Sora's body and the other being Sora releasing his and her heart from his body. Twice, two times. *Lifts up two fingers*
What does this have to do with Xion? Good question, lawlz - well, I'm thinking Xion somehow came from Kairi, using _____ as a body and soul (?). Allegedly, she even sounds like Kairi, correct? Meh -Shrugs-
Idea #2
Xion isn't Aqua, let's just make that clear. However, what happens if she ends up looking like Aqua to an extent? Check this out
Isn't it kinda funny how Xehanort actually shares some similarities to Ansem (SoD) and Xemnas whilst Roxas and Sora, although similiar to an extent, isn't that similar in comparison to Xehanort and his nobody? No? Well, for the sake of this, let's say it is, k? lol
So, remember that infamous quote where Ven requested to be, "Erased?" In KH:CoM, that term is often associated with memories - maybe Ven knew something and didn't want it to slip into the wrong hands? Anyway, we now know he asked Aqua to erase him; I believe he was referring to his memories. Now here's the twist -- Aqua didn't necessarily erase Ven's memory, but rather placed in within Sora for safe keepings (MX wouldn't suspect searching in a kid for w/e he wants, lol).
I'm not sure where memory resides in, but let's say it stays in the heart and soul somehow. Roxas used both Sora's body and soul to be created, and with that, Ven's memory, which I want to say affected Roxas's appearance. Interesting, yes?
So, let's say Aqua lost her memory and it ended up in... hmm, perhaps Kairi. Yeah, I think you see where I'm going with this.
Yup, I'm just throwing these ideas out there. Throw in some of your own, if you want. ^^