Dark Figure

Xehanort's guardian is a dark entity, similar to a Pureblood Heartless, which is used by Ansem, Seeker of Darkness throughout the  series. It is first created by Terra-Xehanort, Ansem's original persona, to battle Aqua in Radiant Garden. The guardian is also temporarily used by Riku when he takes on the form of "Ansem" during his battle with Roxas. The guardian serves not only to protect its user from attacks, but also to fight for him when needed.

Kingdom Hearts Birth by Sleep
Having lost his first battle against Aqua, Terra-Xehanort's great rage causes the guardian to spawn behind him. The dark entity then teams up with Terra-Xehanort to challenge Aqua one final time. Upon Terra-Xehanort's second defeat at Aqua's hand, Master Xehanort attempts to force what remains of Terra out of him by unlocking his heart. However, the guardian reappears and begins to fade, opening a portal into the Realm of Darkness, which it and a severely weakened Terra-Xehanort fall into. Aqua then dives in after them and sacrifices both her armor and Keyblade to save Terra's body.

Kingdom Hearts
After conversing with Sora over the true nature of the heart, Ansem, Seeker of Darkness summons his original persona's guardian and attacks. The guardian reappears alongside Ansem during his second battle with Sora as part of the World of Chaos.

Kingdom Hearts Chain of Memories
After Riku emerges from the twelfth basement of Castle Oblivion, Ansem, Seeker of Darkness appears before him and attempts to retake his body. However, Riku chooses to fight rather than submit, enraging Ansem, who summons his guardian and attacks.

Later, Riku reaches the first basement of Castle Oblivion and uses Naminé's Castle Oblivion World Card to bring forth Ansem. Ansem creates around them a facsimile of Kingdom Hearts itself, and again summons the guardian, who attempts to crush Riku with a swing of its fist. However, Riku is able to defeat both Ansem and his guardian, and he emerges from the battles having conquered his inner darkness.

Kingdom Hearts 358/2 Days
Realizing his loss to Roxas is inevitable, Riku unleashes Ansem's darkness that he had been holding back from within his heart. Riku, now in Ansem's form, uses the guardian to grab Roxas by the shoulders and squeeze him to the point of unconsciousness. This then allows Riku to take Roxas to DiZ as assigned, the only downside being the fact that he is stuck in the form of Ansem, Seeker of Darkness.

Kingdom Hearts 3D: Dream Drop Distance
In the sleeping version of The World that Never Was, Ansem appears to Riku, attempting to persuade him once again to open himself to Darkness and allow his possession. However, Riku chooses to fight and vanquish his inner Darkness, causing Ansem to surround them in Darkness and summon his guardian. After a first battle, Ansem allows the guardian to grow larger and transforming into a greater threat, though Riku defeats this form as well.

Later in the real world, Ansem summons his guardian to prevent Riku, King Mickey, and Lea from rescuing Sora from Master Xehanort. The guardian grabs Mickey and Riku to prevent them from leaving, but is defeated when Donald and Goofy fall on its head after warping in with the Star Shard.

Kingdom Hearts III
In the Keyblade Graveyard Aqua, Ventus and Sora fight Terra-Xehanort in one final battle. When Terra-Xehanort captures the trio and prepares to do away with Aqua and Ventus, the Guardian moves on it's own to intervene and save them. It then breaks the chains binding the trio and puts Aqua and Ventus safely on the ground. Terra-Xehanort is surprised by the guardian's independence and insists that it's heart fell to darkness as it restrains him.

The guardian then manages to rip off the bandages on its mouth and begins to speak with difficulty. The guardian declares his promise to one day return and set things right, revealing that it is the heartless of Terra. With Terra-Xehanort restrained, Sora takes the chance to force Xehanort's heart from Terras body with the power of the Keyblade, while the Guardian sends its heart back into the now vacant body, restoring Terra to his old self at last.

Design
The guardian has an ink-black, muscular, humanoid form with a partially bandaged head that hosts relatively human features, bearing some resemblance to Master Xehanort. It has yellow eyes gleaming above a mouth filled with sharp teeth, as well as two long antennae spouting from the top of its head. A hole appears in between more bandages on the torso in the shape of a heart, much similar to the heart-shaped hole that appears in many of the Pureblood Heartless. Lastly, it has very large hands, its torso coming to a tail-like point that fades away into its host's body.

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 shoulders, faint golden eyes also on the chest. Red tendrils extend from the back, attaching to the guardian with a snake's head-like attachment. Its hands become more claw-like with large, organic sleeves covering its arms, and an extension similar to a scorpion's stinger juts out of its back. The torso also appears more skeletal, with multiple ribs and vertebrae showing.

When transformed in Dream Drop Distance, the guardian retains its base appearance but is much larger, dwarfing Ansem. Its lower body is formed into an orb of darkness which surrounds Ansem, two dark blue wings extending from the sides and curving downward.

Abilities
The guardian has many dark powers with which it assists the one it is paired with in battle. It also possesses great physical strength, impervious to all damage. While it tends to stay behind its host, it can be commanded to detach itself from that host to attack its victims. Able to latch onto these opponents and drain their HP, the guardian can also hit them with bursts of energy spawned from its fists, mighty punches, and even spheres of energy. Similar to a Pureblood Heartless, the guardian's most devastating ability allows it to sink into the ground, creating massive shockwaves every time it re-emerges. The guardian also has the power to shoot disc-like projectiles at enemies, greatly multiplying the host's powers with its own as they double-team opponents to ram them with brute force while shining with a dark aura.

In its World of Chaos form, the guardian becomes more of a defense mechanism than one of offense, able to summon Bit Snipers to distract opponents and to whip its massive sleeves to create large whirlwinds with the purpose of blowing enemies away. It can also drain an opponent's MP. While it is only when the guardian is paired with its host that it is a formidable opponent, it is shown to hold its own quite well without influence, mercilessly attacking the opponent without ceasing.

Ansem (first battle)
When paired with Ansem, Seeker of Darkness, the guardian will immediately go on the offensive in battle, punching opponents if they come too close or bombarding them with dark projectiles if they are out of attack range. On Ansem's command, the guardian will appear in front of him, preventing him from being damaged. However, Aeroga, or attacks such as Ragnarok, can still hit Ansem if Sora runs around to his location. The guardian may also obey Ansem's command to leave him and attach itself to Sora, a "?" occasionally appearing over Sora's head once this occurs. If is pressed when this icon is on the screen, the guardian will latch onto Sora and drain a large amount of his HP. It will eventually return to Ansem in the end, however. Use careful timing of the Dodge Roll ability to dodge this attack, as well as use Curaga when necessary. Guard can be used to earn Tech Points when Sora deflects the dark discs the guardian shoots back at it.

Ansem (second battle)
When Ansem and the guardian face Sora on his own, their moveset will be identical to that of the first battle. However, the guardian and Ansem can now use some new, devastating attacks. The first involves the guardian and Ansem jetting across the battlefield, dealing damage as they collide with Sora. Either Guard or Aeroga can be used to halt the guardian and Ansem. The second is, by far, the most damaging, the guardian sinking into the ground on Ansem's command and creating short-ranged shockwaves as it respawns in an attempt to attack Sora. Dodge Roll can be used to avoid the guardian, as well as the large burst of energy that results in the guardian returning to Ansem.

Ansem (first battle)
During the first battle between Ansem, Riku, and the guardian, Ansem will only shoot bursts of energy while the guardian bombards Riku with punches. In Kingdom Hearts Re:Chain of Memories, however, the guardian aids Ansem as he teaches Riku how to perform a Card Duel. The guardian will only deal punching attacks during this "battle", as well, he and Ansem not taking damage as they did in the original Kingdom Hearts Chain of Memories.

Ansem (second battle)
During Riku's second battle with Ansem and the guardian, the gloves are off. While Ansem shoots bursts of energy, the guardian will punch at Riku, collide with him as he and Ansem fly at him shining with dark energy, or even create shockwaves as it sinks into the ground and reemerges beneath Riku, hitting him several times. In Kingdom Hearts Re:Chain of Memories, it is the guardian who launches the bursts of energy from its fists, also using the dark projectile attack from Kingdom Hearts. The shockwave desperation attack also has the added energy burst effect seen in Kingdom Hearts, as well.

Terra-Xehanort
When paired with Terra-Xehanort during the final battle with Aqua, the guardian will do most of the attacking, creating small shockwaves as it reemerges from dark portals on the ground, slamming Aqua into the ground after carrying her high into the air, should the shockwaves make contact with her. The guardian can latch onto Aqua, at this point rapidly press to free her. The guardian can deal a powerful punch, release twin columns of energy from its fists, and shoot twin energy spheres from each hand. When very little of Terra-Xehanort's HP remains, it will warp Aqua to Terra's Mind. The guardian will attempt to drive Aqua away from the waiting Terra with its energy sphere attack, which causes Blind, but if she makes it to Terra, she can activate the "Dual Limit" reaction command to deal a significant amount of damage to the guardian by teaming up with Terra and shooting it with an immense amount of light energy.