The Heartless (ハートレス, Hātoresu?) are actually hearts corrupted by darkness, lacking a body or soul. They are named "Heartless" for their total lack of emotional capacity and are the most common type of enemy the player encounters in the Kingdom Hearts games. Originally, there were only Pureblood Heartless, black shadow creatures that lacked hearts who were born of pure darkness that usually exist in the Realm of Darkness, and rarely appeared in Realm of Light (the universe of the series). But the experiments by Ansem the Wise and his apprentices into the capacity of the heart lead to the seemingly unintentional creation of Heartless in the Realm of Light.
While researching these Pureblood Heartless, Xehanort and Ansem's other apprentices devised the means to create the artificial Emblem Heartless via the corruption of living hearts. Unlike the Purebloods, which dissolve into black smoke when defeated, Emblem Heartless release hearts once defeated. However, unless the Keyblade was used to defeat the Heartless, the stolen hearts go off into the Realm of Darkness and the Heartless are recreated. Ordinarily, the Heartless are mindless and function on instinct, but they obey those with strong will. However, in worlds closer to darkness, the Heartless are stronger and become uncontrollable. They invade worlds through corridors of darkness, which are unpredictable pathways that interlink the many worlds.