Answer:
well, going with the story, heartless are made when a person totally gives up in the light side of the world. but there are also characters in the kingdom hearts that can change people into heartless. basically, going back 2 kingdom hearts 2, once a persons heart was collected in the sky for kingdom hearts, that body from which it came from would turn into a dusk (assuming they had a strong heart). there soul however turned into a heartless. hope this helps a little