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Ventus’ heart is fractured before the beginning of Birth by Sleep. Here, he is taken to Destiny Islands by Xehanort where he (Xehanort) is supposed to live out the remainder of his life (no such luck).
In Ven’s “dive to the heart”, he meets Sora who shares his heart with Ventus, allowing it to fill in the gap of Ven’s missing darkness (Vanitas).
Now at the end of BBS, Ventus battles Vanitas and practically destroys his heart in the process of preventing the creation of the x-blade. Unconscious, Ventus is then taken to the Land of Departure by Aqua where she turns their old home into Castle Oblivion. Only Aqua can find Ventus and anyone who travels to this castle will be “lost to oblivion”.
Eventually, Ventus meets Sora again, and he asks if he can stay with Sora until his heart is completely mended... which Sora allows.
10 years pass and Ventus’ heart is ‘completely mended’... so to say. However, since his body is trapped in Castle Oblivion, it has no body to return to when Sora “unlocks” his heart, releasing the Princesses of Heart, Kairi’s heart, and (of course) Ventus’.
Through this process, Sora becomes a Heartless, albeit not for long. However, a Nobody is still created - two actually: Roxas and Namine.
Ven’s heart fled to the empty shell left by Sora which was Roxas, which is why Roxas was able to feel emotions compared to the rest of Organization XIII. It’s also why Sora’s Nobody took on Ven’s physical appearance opposed to Sora.
Now I don’t remember where exactly I read this, but the reason why Axel was able to have feelings too was because he hung out with Roxas so much. Roxas is a Special Nobody (like Namine) in that the Organization members that were around him would begin to feel too.
Don’t believe me?
Let’s compare Axel’s personality in Chain of Memories to those when he’s with Roxas in 358/2. In CoM, he’s literally a Nobody, not caring as he practically smites Vexen and Zexion (...he kills Zexion, right?). After he lets Namine free, he laughs to himself, saying (in the original CoM) something about “putting on one hell of a show”.
I feel the reason he could laugh and even consider letting Namine go was because he was close to Namine. This is the same case with Roxas in that he feels empathy (sympathy, whatever) when he’s around the two Special Nobodies.
Originally, Namine was just an empty husk, having no emotions. But over the course of CoM, she feels regret, and by the end she even sheds a few tears in regards to what she did to Sora.
Accompanied by Sora’s later claim: “you need to have a heart to cry”, this leads me to my next point:
Nomura claimed that Nobodies eventually grew hearts of their own.
So this leads me back to Axel, who more than likely had his own heart by the end of 358/2. In one scene in KHII final mix (or 2.5), Axel practically argues with Xemnas when the latter says he wants Roxas destroyed, or else he would turn Axel into a Dusk.
If Axel didn’t have a heart, he wouldn’t have protested in such a way.
To add more support, Roxas isn’t even present.
But I’m assuming the fandom already knows this so let’s get back to Roxas.
If Nomura said Nobodies (of Organization) can grow hearts of their own, then that means Roxas developed one as well. Meaning he had two hearts inside him, not just one - Ven’s and his own.
Nomura later went on to say Roxas is his own self and that Ven’s heart had little to no influence over him aside from enabling him to have feelings in the beginning of Days. Oh, and his ability to dual wield (Oblivion and Oathkeeper). Many people assume this is Xion’s influence, but it’s Ventus.
So technically, when Roxas returned to Sora, people say Ventus returned to Sora as well.
However, Namine claims in Re: Coded that while she was repairing Sora’s memories, she came in contact with Ventus’ heart. At this time, Roxas was either with the Organization, or he was trapped in the Virtual Twilight Town with altered memories.
Does this mean Ven’s heart returned to Sora when Riku captured Roxas? The only time Namine could have met Ven is when Sora was asleep and Roxas was on his own.
This leads me to assume that it is indeed possible (and very likely) for Roxas to have had Ven’s heart in the beginning of Days and eventually grew his own. Would that mean Namine met Ven only when Roxas was near Sora’s body? I could see why Ventus’ heart could return to Sora for Namine to make that claim.
By the time Roxas draws Oathkeeper and Oblivion when he fights Axel, he’s already summoned them before, so I don’t think he would need Ven’s heart in order to summon them again. Also, when he fights Sora in the Dive to the Heart, he’s dual-wielding - again. Considering that Roxas had merged with Sora sans arguing, one can’t really use this to argue that Roxas still had Ventus’ heart because... well, they’re all back together in Sora’s heart, so that wouldn’t make much sense. But regardless, it shows that Roxas didn’t need Ventus to dual wield the third and (possibly) second time he summoned Oathkeeper and Oblivion.
This is just a theory of mine that I’ll probably stick with when I write for Kingdom Hearts, and if anyone has better suggestions supported by canon, then feel free to let me know.