The Heartless are beings of Darkness, born when a Heart is consumed by it and they are the recurring major antagonists in the Kingdom Hearts videogame series. Pureblood Heartless are natural creatures that balance out the Light within the world, however Emblem Heartless are unnatural and throw the worlds into chaos as they actively seek to destroy the Light. Although Heartless are controlled by instinct, and will seek and capture hearts they can find, they will ally with whoever stronger than them.
(Spoiler) In rare occasion, if a person with strong heart becomes a Heartless, the empty shell he left behind will begin to act on it's own. A shell who's heart had been stolen, a spirit that goes on, even as its body fades away from existence.