Guardian

The Guardian is an enemy that serves as a defender and a provider of in-battle support to Ansem in the first two battles against him in Kingdom Hearts.

Story
Ansem had the power to project an unknown Heartless from behind his back. It served as a familiar to Ansem, and would do most of Ansem's attacking and defending when battling Sora. During the first two battles against Ansem, Ansem would yell "Come, Guardian!" before it threw itself in front of him as a shield, hence its unofficial name. Eventually, the Guardian transformed into the World of Chaos. The Guardian resurfaced during Kingdom Hearts II: Final Mix+, when Riku took the form of Xehanort's Heartless to defeat Roxas in the World That Never Was. During this state, Riku loses control and the Guardian pummels Roxas senseless before Riku can stabilize his heart. It appears once again with Ansem in  Kingdom Hearts Re:Chain of Memories in Riku's story, but it wasn't real it was just a dark memory in his mind.

In Battle
The Guardian can attach itself to 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 like strike raid. 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 its design. It is believed to be a Pureblood heartless, as it bears no emblem, and it is part of Xehanort's Heartless.

Rumors
As stated, the Guardian posses many similarities to the Neoshadow and is considered another form of it. It is unknown whether the Guardian is an actual part of Xehanort's Heartless or not, but this would seem likely, for when Sora changed into a Heartless, his form was a Shadow, of which the Guardian is believed to be another form. This theory is also supported by the fact that Riku was able to summon the Guardian in his fight against Roxas, though this may only be a result to the possibility that Riku spent time with the Heartless when he was possessed by Xehanort's Heartless. If the Guardian was not connected to Xehanort's Heartless, it is possible it still exists.