Heartless are essentially hearts that have been corrupted and enveloped by darkness, taking a physical form. It’s unknown exactly what form a heartless will take, or what causes mutations in species of Heartless.
The Heartless have existed for as long as evil existed in man’s heart. It was the emotions that man carried with him that gave birth to the heartless. Greed, lust and power are emotions traditionally associated with the darker side of mankind. It is when one gives into these emotions that the Darkness will overcome his heart, envelop it, and take the heart for its own to use for its own malevolent deeds. Though Darkness is not inherently evil, those with small willpower are easily overtaken by it. There have been many who have tried to study the process of becoming a Heartless, however most have also succumbed to the Darkness due to the concentration of and exposure to it. The few that managed to brave the dangers of such studies, however, were met with no spoils, as they could not come to any conclusion as to how the process is actually done. Some say it is better to leave such studies alone out of fear of the legends of such people who once knew the secrets of creating such Heartless.
As Darkness and Light are opposites, they are inevitably drawn to each other. Heartless are drawn to strong Light affinities, like a moth to a flame. Likewise they are drawn towards those with strong Dark affinities, seeking kinship. Heartless nearly always travel in groups. Strength in numbers and vicious yet deceptively cunning battle styles are commonplace in the more advanced orders of the beasts. It is theorized that those with Light affinities are drawn to the Darkness of the Heartless, but it is difficult to prove this claim.
Falling Into Darkness
While having an affinity to the Darkness does not necessarily deem you the foremost candidate of becoming a Heartless, it will certainly not help your case. Those that would seek out Darkness out of lust for its power will indeed fall victim to the power, transforming into a being of pure Darkness. The same outcome could be achieved by other means of course, such as being in an area with a high concentration of Darkness, or having your heart taken by a Heartless. Whatever the case may be, an unnatural imbalance of Darkness in ones heart can likely take control of them. Although researchers could not find out exactly how the process of becoming a Heartless works, they have found a connection between their physical Heartless manifestation and their Heart. It was reported that most individuals with a strong Heart, regardless of Heart Element, more frequently manifest into powerful Heartless with some being even capable of speech. It can be said inversely for those with weak Hearts becoming Lesser Heartless, such as a lowly Shadow.
Most regular Heartless are vicious in the fray of battle, clawing and attacking anything that they perceive as a threat. There are a few more dangerous Heartless, however, who are in tune with the mystical arts. In defeat to a Heartless, it's possible that you could lose your very heart and become one of the walking shadows. A victory however grants you much more exciting results. Like all victories, you will almost be certain to reap the spoils of your battle from the said defeated Heartless. Some Heartless tend to leave Essences behind in the wake of their defeat. The other result of your win is however much more complex.
The act of purifying, as complex as it may be, is simple enough in context. To do such an act, it is necessary to first exhaust the Heartless of its physical energy. It is a common misconception that in order to do such an act you require a Keyblade. While a Keyblade would most definitely aid your efforts against it, Heartless are vulnerable to any sufficiently powerful magick or physical attack. Once its energy has been exhausted it is up to the being that the Heartless once was to accept its fate. While most common Heartless will accept defeat at the tip of your blade some may not.
Regardless of however the Heartless accepted defeat, one thing remains the same. After their defeat, the once corrupted Heart that was clutched in the hand of Darkness is released. While it is speculated that all Hearts saved from the grasp of the Heartless return to "Kingdom Hearts," there has not been any solid evidence to back up this claim. Most information on "Kingdom Hearts" has only been mentioned in ancient texts dating back to what they claim to be the very day man was born. The only person to embark on a quest to reveal more on "Kingdom Hearts" was D Illuminate. His whereabouts are currently unknown.
Types of Heartless
Most Heartless are members of the basic set. These are mostly instinctual, though some show the ability of higher cognition. However, this is not as deep as the Heartless can go. There are surely more types of Heartless to be added to this catalog as they are discovered.
The Greater Heartless is thought to be formed when one dies but still has a great attachment to this world. As such, their heart would refuse to join with the Heart of Hearts, and would stay earthbound. Great darkness would gather to give this Heart tangible form. Unlike regular Heartless, who have their appearance altered when they are formed, Greater Heartless maintain their original form. These are capable of speech, and are thought to still have the mind of their former self, but this cannot be proven. Greater Heartless can only be defeated if their attachment to the physical realm is severed. Greater Heartless show no fear towards the Keyblade. One such Greater Heartless is Braghn the Jailor.