...I think it's the other kind. But really, they aren't human anymore so who knows. They're beings called Nobodies. Most don't look human, but these guys do.

They do. When somebody loses their heart, darkness usually takes the heart over. And that's what a Heartless is. Darkness or darkness taking its own heart captive. Heartless taking other hearts ensures the same thing happens again and again. But when a Heartless forms, the body and soul have to go somewhere, right? And that's a Nobody.