When a person loses their heart to darkness, two things happen. There's the empty body that's left over and then the manifestation of the darkness in their heart. The darkness that's left becomes a creature we call a Heartless.

They kind of just go from world to world, looking for more hearts.

Sometimes, though, the body that's left behind still has the willpower to move. All it has are memories without a heart. Nobodies don't act like Heartless, just kinda going here and there - but they don't feel anything either.

Well... Until a heart sort of grows back.