Possessor

The Possessor is a boss from Kingdom Hearts 2.

Upon reducing the Thresholder's HP to 0 and using the Reaction Command, Sora purged the Keyblade into the Thresholder. This released the life force of the Thresholder: A small, cute, harmless, pure Heartless known as the Possessor--the true evil behind the Thresholder. It would float about helplessly before returning to the door, bringing the Thresholder back to life. Sora could not defeat the Thresholder until the Possessor's HP hit 0. It then disintegrated.

Along with the Thresholder, the Possessor is the first Heartless boss in the game, and is the mini-boss of Beast's Castle.

Other Appearances
If the player looks closely when they defeat a Gargoyle Knight or Gargoyle Soldier, they may notice a dark wisp rising from the statue when it is defeated. This too is a Possessor (although not as powerful as the Thresholder; it dies immediately after being released). Additionally, enemies such as the Hydra use an attack that makes small wisps rise from the ground and attack Sora. Although it is likely that these are not Heartless (as they are used by normal monsters such as the Hydra), they bear a very close resemblance to the Possessor, even including the eyes.