Well, I'm not actually disagreeing with you Mikazuki, I agree that Unbirth are pure souls with no body and/or heart to speak of. It's just process of elimination as to what happens to the souls after they are "born". We know what happens to a soul born into the world of light: it becomes a person, normal everyday person. But what happens if the soul is born in darkness? If "born" is a soul brought into the world of light, a soul brought into the world of darkness is the exact opposite, truly an "Unbirth", which is the opposite of birth in every sense of the word. Also, something else just occured to me. Remember back to KHI, when the final battle against Xehanort's Heartless was about to start. He said," All souls begin in darkness, and all so end". I don't know if that statement is KH canon or not, but if so, that solidifies the idea that Unbirths are in reality souls that were never born, and having never left the great darkness from whence they came, became those twisted creatures known as Unbirth.