Shadowy Creatures born from darkness.
They can vary. But mostly emotionless things bent with one goal... steal hearts.
Varies depending on which one you're dealing with, but they mostly use scratching. They can also turn into shadows and sneak along the floor.
Created from the darkness within a person's heart, they will always exist a long as there is darkness. If they do so manage to steal a heart, whatever the heart was stolen from becomes a heartless.