The Heartless are Sora's archenemies and shadowy demonic creatures created by the forces of evil. They are the remaining body of someone who has lost their heart and has nothing less to turn into but darkness.
The original type of Heartless, and the most common. They are completely black with piercing yellow eyes and antennas, and have the ability to teleport by sinking into the ground and moving incredibly fast. Sora faces these species many times during his adventures, especially near the end. Examples include the Shadow, Darkball and Darkside Heartless.
The biggest variety known, Emblem Heartless are plastic-looking, colored Heartless, modified by "Ansem" to gain more power when collecting hearts. They represent the biggest part of the Heartless enemies and bosses in both games. Examples inlude the Trickmaster, Shaman and Groundshaker Heartless.
Normally, the Emblem Heartless sometimes appear as more authentic versions to the World's nature and laws, even going as far to appear only in that specific world (example: the Fortunetellers are Arabic-themed Heartless, thus only appearing in Agrabah, and etc).
Role in the series
Fagin has the ability to control the Heartless due to his gift from Malefor, as do many of the Villain Leagers in the series. They will return again many times more under the command of Hades, Gaston, Madam Mim, Ursula, Cruella De Vil, Captain Hook, and some others.