Guardian

The Guardian is an enemy from Kingdom Hearts.

Story
Ansem did not battle Sora alone in the End of the World, he used a familiar Heartless known as the Guardian. It served as a shield for Ansem during the battles against Sora. It would leap from behind Ansem to block an attack. Eventually, the Guardian transformed into the World of Chaos. The Guardian resurfaced during Kingdom Hearts 2: Final Mix+, when Riku used the dark powers to defeat Roxas. It no longer makes an appearance after being used to subdue Roxas, as Riku, who only used the darkness as his last resort, elects not to summon it any longer for fear of the darkness already eating at him.

Due to its immense power and ability to transform into a much more powerful version, it is possible that the Guardian is the most powerful of all Heartless. This stems from the fact that some believe the Guardian to be Ansem's true essence, as it primarily does all the fighting between the pair and even appears to Riku after he opens his heart to Ansem's darkness. It is unknown if the Guardian will ever appear again or if it's gone for good.

In Battle
The Guardian can possess Sora during the first two Ansem battles. It has the ability to sink into the floor and rise out in a burst of dark energy, similar to Cerberus' dark attack. It can also fire disks, which appear to be razor-like. When possessing Sora, the "Freeze" command appears frequently, replacing the "Attack" command. Selecting "Freeze" causes the Guardian to hold Sora in place, allowing Ansem to attack the defenseless Sora. It's better to attack when possible and heal when necessary; as the Guardian damages Sora occasionally when in possession.

Similarities
The Guardian has often been associated with the Shadow, as it bares many resemblances to the Neoshadow, possibly even inspiring it's design. It is believed to be a "pure" heartless, as it bares no emblem. However, It was able to produce an emblem once Ansem was defeated, leading to much uncertainty.