The Heartless are a type of creature appearing in the Kingdom Hearts series. They are monsters born of the darkness in people's hearts.
Heartless have always existed in the Realm of Darkness as offsets of people's darkness, and used to rarely, if ever, appear in the Realm of Light. However, roughly a decade ago, the apprentices of Ansem the Wise, led by Xehanort, performed multiple experiments into the nature of the heart. While pursuing the research, Xehanort discovered Heartless and with them, found the heart of his world. Based on them and further study of them, he created artificial Heartless and marked his creations with an emblem.
After expelling Ansem from the world into the Realm of Darkness, Xehanort and the apprentices continued their research and eventually gave their hearts to the Darkness, turning into Heartless and Nobodies. Xehanort's Heartless then began to use the Heartless to take over the worlds. The Heartless destroyed most of Radiant Garden, transforming it into Hollow Bastion, then left to devour the hearts of worlds.
By the beginning of the original Kingdom Hearts, Heartless had become common on many worlds, and had taken the hearts of many as well.
Common Heartless are mindless in nature. Devoid of a soul or body like Nobodies, and constructed only of slivers of darkness from a person's heart, they are driven by pure instinct alone. Their only goal is to devour hearts, leading them to two objectives - steal the hearts of a world's inhabitants, and find the heart of a world.
The only thing that inhibits them from fully performing this task is the presence of a Keyblade, which can destroy them. The Heartless are naturally drawn to the heart of a Keyblade Wielder, and will spawn specifically to attack them. After Alexander Karsath's interference, the Lockshield, which mimics the Keyblade, also becomes a target.
As Heartless are mindless, they do not have direction of their own. This may be taken advantage of by somebody with a certain level of control over darkness and a strong will, who can also use that control to command the Heartless. Maleficent, for a long time, held control over the Heartless in this fashion, but was usurped when a more powerful individual appeared, and the darkness was quick to devour her heart when she showed weakness. Powerful Heartless and Nobodies hold an advantage over regular people in this respect, as their hearts cannot be devoured by the Heartless.
To travel between worlds, the Heartless use Dark Corridors. When they devour the heart of a world, the remains of the world become new ones: for example, the End of the World, the endgame of the original Kingdom Hearts, is constructed of many pieces of other worlds.
Heartless come in two major categories - Pureblood and Emblem.
Pureblooded Heartless are naturally-occuring Heartless, born of the darkness in people's hearts. They are constructed purely of darkness with no other human elements. They spawn in areas that are more saturated in darkness, and appear to make their home in the Realm of Darkness. Shadows are the most common Purebloods and appear in most worlds.
Emblem Heartless, in contrast, are artificially-produced Heartless, originating from attempts to recreate the enveloping of a heart by darkness during Xehanort's experiments. Emblem Heartless are produced from stolen hearts and release the heart they are constructed from when they are destroyed. They are found in every world in the Realm of Light and are distinguished by the emblem they sport on their bodies (hence their terming). There are also a wider variety of them in existence.
Heartless range in appearance from small to large and come in multiple forms, including those of objects, animals and humanoids. Pureblooded Heartless are more likely to appear as humanoids and generally do not sport the brighter colours of Emblem Heartless, with some special exceptions. Emblem Heartless, meanwhile, come in a much larger range of colours and shapes, and often reflect elements of the world they appear in.
The Heartless Apprentices are extremely powerful Heartless which retained humanoid form. Of them, Xehanort's Heartless, who branded himself as Ansem, Seeker of Darkness, was the final boss and the major antagonist of the original Kingdom Hearts. The other apprentices target Alexander beginning with The Interference II: Curse of the Crimson Elixir, with each apprentice selecting a world to attack Alex in.
Of all of the Apprentices, Even so far is the trickiest, having gone to the effort of producing new Emblem Heartless to take advantage of and augment fixtures in Haven City. This includes a large spider Heartless that can modify guard turrets to become more powerful.
During The Interference II, Alex's sister Erica is turned into a Heartless by Dilan. Like the Apprentices, she also retains human form, presumably due to a large swell of darkness within her heart. It is unknown whether she truly selected to give up her heart, if she was goaded into giving it up, or if she was turned into a Heartless by force, although the second is most likely.
Sora and Alex's Anti-Forms are both reminiscent of Heartless. Sora's is a reflection of the time that he became a Heartless in the original Kingdom Hearts, and doesn't currently affect the state of his heart. Alex's, meanwhile, is a true engulfing of his heart by darkness, with the capability to turn him into one if it gets out of hand. Sora's level of self-control during the form is unknown, but Alex's self-control fades.