First, the easier of the two: Roxas.
As has been stated in the Ansem Reports, I believe that Roxas is more of a conventional Nobody. He is Sora’s body and soul. This makes the most sense.
But a not-so-unexpected twist was revealed in Birth by Sleep: Ven’s fractured heart dwells within Sora. Now, I’ve seen some looming tension in the Spoilers section. Some believe that Roxas has Ven’s heart, others believe that it still rests with Sora’s, and Roxas is simply using it remotely.
With which side do I ally myself? Those who say he has Ven’s heart. My reasoning is twofold:
1. Look at the effect the Heart Unlocker had on Kairi’s heart. It separated her heart from Sora’s. Why would it not do the same with Ven’s heart? But, remember, the Heart Unlocker doesn’t necessarily release hearts, it activates their potential (just look at Maleficent). For Kairi’s heart, this meant separating from Sora’s heart and returning to her body. But Sora’s heart was kicked out and consumed by darkness. Why? Because of the effect it had on Ven’s heart. While Ven’s heart remained dormant in Sora over the years, the Heart Unlocker allowed it to be activated, so to speak, through Roxas, which effectively pushed Sora’s heart out.
2. Note Roxas’ appearance. He looks exactly like Ven. Why? Well, it wouldn’t make much sense if he only had Sora’s body and soul. Sure, he might be remotely accessing Ven’s heart, but in that case, wouldn’t he look more like a mix between Sora and Ven, as opposed to completely like Ven? My recent exposition on the heart asserts that an active heart dictates the appearance of a vessel. Hence, with Ven’s heart within Roxas, he assumes the identity of Ven. And remember, Nomura said that both Sora and Ven are Roxas’ “original selves.” If Roxas didn’t have Ven’s heart, how could Ven be considered one of Roxas’ original selves?
That to me, is a pretty open and shut case, though for many of you it isn’t. I know what you’re thinking. How is Roxas still a Nobody when he has a heart?
The answer is pretty simple. It’s not a whole heart.
Could Ven sustain himself on a fragment of a heart? Obviously not. So, to be a truly complete being you need, no surprise, a complete heart. Roxas was able to function with only a piece of a heart unlike Ven because, as a Nobody, he doesn’t need a heart to function in the first place. It’s supplementary, but not enough for us to regard him as complete. I suppose you could argue that with a fraction of a heart he could almost be seen as a Nobody/whole being hybrid, but that’s taking it a bit far. I believe completeness follows the all-or-none principle. You either have it or you don’t.