Question...[Spoilers]

[SeeDRankLou]

What one could deem by literary license, is that a Heartless is whenever one is split into its respective halves -- if you have a Nobody, then you are the Heartless. One could think that "Sora" played at the end of KHI and most of KHII, is a Heartless, but in Human form. The justification is that Ansem Darkseeker was also a true Heartless, but had a human form, and that Roxas does not fully reunite until Memory Skyscraper, which means that Final Form is Sora, as a whole, and fully wielding the power of the Keyblade as its Master should be. Anti-Form could also be used to justify that Sora cannot keep up the Charade longer, and he falls to the Dark thoughts within his mind, and unleashes power, unholy, and not with aid of the Keyblade. He also doesn't use recharging Orbs.

Of course the argument against the theory, is the exception of Kairi. She lost her Heart, but no Heartless formed, and also because she has no Darkness inside of her. This is untrue of Sora, who DOES have Darkness in him; he is no Princess of Heart.

When a heartless is formed, the amount of darkness that was in that person/being's heart has something to do with them being a heartless. When Sora turned into a heartless, he had very little darkness in his heart, but not none. So that amount of darkness affected how he changed when he became a heartless. Xehanort had become obsessed with the darkness and had taken it into himself completely, weilding the darkness like no one else. So I think that when Xehanort turned into a heartless, his entire being was already darkness, so it was like he was already a heartless, he just needed to lose his heart. The darkness didn't change him because that's all there was. Kairi on the otherhand was the opposite but had the same outcome. Kairi is one of the seven princesses of heart, which means that there is no darkness in her heart. So when her heart left her body and she became a heartless, there was no darkness in her heart to change her appearance, so her appearance didn't change. I know this whole situation is up for debate, but that's just how I see it.

However, I will say that I don't think Sora is a heartless anymore. Like Tavrobel said, when Sora unlocked Kairi's heart, his heart went with hers, and when she protected him from the heartless, she somehow gave him his heart back, and he became himself again. Though incomplete because of the existence of Roxas, definately not a heartless anymore.