Special Nobody = can be just about anything, is my view on it, as long as it was made during the Heart's departure from the Body but is not the Heartless/Heart.
I'm not saying she's an Unbirth or something, but not quite a "Body + Soul" :3
which ties in a tad better imo with her being Aqua related. As much as I like the notion Xion might've been born as Kairi's Nobody on DI, it would still leave her with even almost less than Namine had. Namine at least really had two Hearts leave a single Body to explain two beings being born during the process. Xion would've been the "second" along with Kairi's own Vessel from a process that only had one Heart leave a Body @-@;;;