Heartless

Heartless are the darkness in hearts of living beings given shape and form. They have a thirst for hearts, and are most attracted to pure hearts (i.e. the hearts of good people). Their existence became known after one was created by Ansem, as revealed by his reports. Emblem Heartless were later created by Xehanort. (Kingdom Hearts, Kingdom Hearts II)

Origin
Though the origin of the Heartless is not explicitly described or shown, there was one point where there was not a single person with darkness in their heart, and only light existed, and as such all the worlds were connected. However, people began to grow selfish over the light, and thus darkness was born in their hearts. This darkness eventually consumed the light, but the children of light were able to craft new worlds, though they would remain separate. However, the greatest light would reside in the darkness, and until the day comes when that light breaks through the darkness, darkness would continue to thrive.

An unknown, but large, amount of time later Ansem the Wise, the ruler of the Radiant Garden, discovered the threat of the darkness. In order to combat it, he and his six apprentices, Xehanort, Braig, Dilan, Even, Aeleus, and Ienzo, tried to find a way to prevent the darkness from consuming the world, performing many experiments on the human heart. However, Ansem the Wise began to see the immorality of what he was doing, and ceased his experiments.

But Xehanort and the others continued with their experiments, discovering the Heartless and their nature. They constructed a large factory that could reproduce the creation of Heartless, thus making Emblem Heartless. Thus, the Heartless, once a minor threat, turned into a force that threatened all the worlds. (Kingdom Hearts, Kingdom Hearts II)

Nature
Heartless, Emblem or Pureblood, possess several key characteristics that define them as Heartless. All Heartless are created when the darkness that resides in a person's heart consumes him or her, thus giving the darkness shape and form (this process occasionally forms Nobodies, which are born from the body and soul left behind when the heart is lost). Being born from darkness, they are mindless and master-less, and their only goal is to find hearts, wishing to consume them and consequently create more Heartless. But what they desire above all are the hearts of worlds, and thus they enter worlds in search of the way into the hearts of the worlds. (Kingdom Hearts, Kingdom Hearts II)

Biology
According to an analysis performed on several Pureblood specimens by Ensign and Crewman William Telfer there is no evidence of a heart, but of a very muscular chamber that utilizes pressure differentials to circulate the blood. The blood of the Heartless appears to be a rather viscuous fluid in its natural state.

That same analysis confirmed Heartless have a hide with the consistency of moderately tough leather and that there was no evidence of genetic engineering. The autopsy also revealed evidence of an anaerobic chemical process that suggests the creatures could survive in a vaccuum.

The analysis also revealed that the Heartless brain was almost that of a sentient being. The presence of antennae and lack of vocal cords in the specimen caused Wildman and Telfer to hypothosize that the creatures communicate telepathically or are part of a hive mind of some sort. (Star Trek: Voyager -Dark Realm)

First Contact with Voyager
First contact with the Heartless occured when the Federation Starship USS Voyager (NCC-74656) encountered a Vaadwaur fighter adrift in space. The away team beamed to the ship under the command of Commander Chakotay found evidence of a close quarter battle that slaughtered all but one crewman on the Vaadwaur ship.

A half mutated Vaadwaur survivor was discovered and beamed back into Sickbay. The mutant had three times the strength of a normal humanoid and seemed to be in a less than rational state. While Captain Janeway attempted to converse with the mutate in Sickbay the Vaadwaur completely mutated into a Heartless and broke through the forcefield and attacked her. Janeway was saved from injury by the timely actions of security officer, Lieutenant Ayala.

When Voyager reached a spatial rift detected by Astrometrics and began investigating it came under attack by several Heartless ships. During that time, the Delta Flyer was separated from the ship and sent into the rift.

In what would become known as The Away Mission from Hell, the Delta Flyer's crew would encounter the Heartless time and again as they attempted to thwart the ambitions of Armus and Biff Tannen. (Star Trek: Voyager - Dark Realm)

Under Armus' Thrall
When a Nobody attempted to absorb its Heart, Armus absorbed its dimensional travel abilities. After making contact with Biff Tannen, following its first interdimensional jump, Armus began to form an army of Heartless and Nobodies, the drifting refuse of Organization XIII's failed attempt to regain their hearts. (Star Trek: Voyager - Dark Realm)