Well for some time I've been thinking that there was gonna be a 3rd breed of enemy such as heartles and nobodies. Well here is one thing, though not much that makes me suspect it.
'-The name of the knight that looks like Roxas is named Ven. His name is in relation to the sky, similar to Sora. Nomura also hinted at his possible connections with Roxas, and in turn, Sora.'
Well if he looks like Roxas/Sora and is connected to them, he's probably a different form of them.
I also think there is a third type because, Roxas draws a third symbol in the Mansion In Twilight town, the other 2 symbols of course being the heartless and nobody insignias (did I spell that right)
Here's a picture