Supposedly, when someone surrenders their heart to the Darkness, if their will is sufficiently strong, they become a Nobody. If I understand correctly, this means that the Nobodies are the bodies, the shells of someone who lost their heart. Doesn't this mean that the Nobodies are the creatures that are 'Heartless'?
For all the fact that the Ansem Reports in KH1 claimed that the Heartless are literally 'those without hearts', this seems to be incorrect. When Sora (or Goofy, or Donald, or Sephiroth, apparently) kills these little critters you get a floating pink, glowing heart-shaped thing rising out of them as they disintegrate. Axel says in KH2, "Every Heartess slain with that Keyblade releases a captive heart." Later, Saix says, "The rage of the Keyblade releases those hearts." These comments, and my own observations, have led me to believe that the Heartless are classified incorrectly, and they are instead a manifestation of a heart imprisoned by Darkness. How this mistake could have occurred, I have no idea, but I guess that after nine years of the so-called 'Heartless' running rampant through the worlds, I guess that it would have been too confusing to change it.
In that vein, it has puzzled me that Goofy and Donald are apparently also able to release the hearts as well, since they aren't wielding Keyblades. Sephiroth, in KH2, was shown wasting Heartless - pink glow-y things floated off to God-knows-where when he did that. This seems to be a contradiction in what Axel and Saix said. I don't know whether there was something that specifically said this, but I seem to remember someone saying that Keyblades were the only weapons that could slay the Heartless. Or maybe it was that the Keyblade was only the weapon that the Heartless feared. I can't remember.
Anyway, this is all I can think of right now.
Hi everyone! My name's RC, and I'm here to get a theory off my chest that I've been pondering since I started playing KH2. Here we go.