Xehanort's Guardian is a special dark entity used by Xehanort, Xehanort's Heartless, and Riku throughout the Kingdom Hearts series. The Guardian serves not only to protect Xehanort from attacks, but also to fight for him when needed.
Story
Kingdom Hearts Birth by Sleep
After losing to Aqua in the first half of their battle, Xehanort's great rage caused the Guardian to take shape behind him. Xehanort rechallenged and lost to her yet again, the Guardian dissolving and dragging Xehanort into the Realm of Darkness. Aqua then proceeded dive into the dark pool before her after him, sacrificing her armor and Keyblade to save Terra's body.
Kingdom Hearts
After conversing with Sora over the heart, Xehanort's Heartless proceeded to summon his Guardian, preparing to battle Sora due to his naïvity. It later appeared alongside him during Xehanort's second battle with Sora, again transforming and increasing in size to become part of the World of Chaos. It, along with the rest of the World of Chaos, was destroyed at Sora's hand.
Kingdom Hearts: Chain of Memories
In Kingdom Hearts Re:Chain of Memories, Xehanort's Heartless appeared in front of Riku in Castle Oblivion in an attempt to possess Riku as he had before. Enraged when Riku chose to fight rather than submit to him, Xehanort again summoned the Guardian behind him. The Guardian attacking Riku after his transport to the abyss, its mighty fist barely missed Riku, the boy's final battle with Xehanort ensuing.
Kingdom Hearts 358/2 Days
After realizing his loss to Roxas was inevitable, Riku unleashed the dark power that was Xehanort's Heartless that he had been holding back from within his heart. Once assuming the psuedo-Xehanort's Heartless form, Riku used the Guardian to grab Roxas by the waist and squeeze him to the point of unconsciousness. This then allowed Riku to take Roxas to DiZ as assigned, the only downside being the fact that he was stuck in the form of Xehanort's Heartless.
Strategy
Kingdom Hearts Birth by Sleep
The Guardian will fight for its master, grasping onto Aqua and carrying her high into the air, slamming her against the ground before returning to Xehanort. It may also send electrical surges forward from its fists. As it will do all in its power to prevent Aqua from attacking Xehanort, it may also perform a punching attack to drive her off. It may also fire dark energy balls before vanishing for a time and allowing Xehanort to land a few quick hits with his Keyblade. When the Guardian latches itself onto Aqua, the player must rapidly tap the 
button to escape a devastating attack. Dodge Roll can be used to avoid many of the Guardian's attacks. The Guardian will also be transported along with Xehanort and Aqua to a bright realm through Terra's remaining consciousness as it fights Xehanort from within. Here the Guardian will shoot an occasional energy ball, and when you close, a physical attack. You can return Aqua to the real world by hitting Terra or using the reaction command to channel light and release it.
Kingdom Hearts
The Guardian fights this time for Xehanort's Heartless. It will utilize the same tactics as past encounters, also able to fire disks of dark energy from its palms. When Xehanort's Heartless commands the Guardian to "come," it will temporarily defend him from all damage. When it latches onto Sora, a "?" will appear above his head. The Guardian will continue to attack Sora, changing the Attack option to Freeze. If Freeze is selected, it will drain a large portion of HP. When Xehanort's Heartless shouts "Take this!", the Guardian will create an energy shield before Xehanort that damages Sora as Xehanort flies into him. The Guardian also participates in one of Xehanort's most powerful desperation attacks, rising up from the ground several times beneath Sora until the attack ends in a surge of dark energy.
When part of the World of Chaos, the Guardian does no damage to the player, merely creating strong gusts of wind to keep Sora away from Xehanort.
Kingdom Hearts: Chain of Memories
The Guardian will perform the shock attack, punch tactic, and Xehanort's most powerful desperation attack when fighting against Riku. It should be noted that in Kingdom Hearts Re: Chain of Memories, it also can fire the dark energy discs at the opponent.
Appearance
The Guardian has an ink-black muscular, humanoid form with its partially bandaged head with relatively human features, yellow eyes gleaming above a mouth filled with sharp teeth and two long antennae spouting from the top of the Guardian's head. A hole appears in-between more bandages on the torso in the shape of a heart.
While part of the World of Chaos, the Guardian mutates, its antennae becoming long horns, the rest of its body distorted by the darkness to look more muscular and ominous. Blue orbs rest on its shoulers, faint golden eyes also on the chest. Red and tendrils extend from the back, attatching to the Guardian with a snake head-like attatchment. Its hands become massive wings, an extension similar to a scorpion's stinger juts out from its back. The torso also appears more skeletal.
Gallery
- Terranort Guardian.png
Xehanort and the Guardian, moments before both fall into the Realm of Darkness after their battle with Aqua.
- Guardian.gif
The Guardian as it appears in Kingdom Hearts: Chain of Memories.
- WorldofChaos.png
The Guardian transformed as part of the World of Chaos.
- Xehanort Heartless.jpg
Xehanort's Heartless with the Guardian.