"Indeed." Xemnas said, confirming her first statement. "Heartless are the darkness within a person's heart, born when one loses his heart to the darkness. With a few rare exceptions they are non-sentient, shadowy creatures that seek to consume the hearts of others, which in turn creates another Heartless."

His own Heartless was one of these exceptions, and so was the one of the Keybearer who, against all odds, managed to return to his original form.