Dark Figure

The One Behind, also referred to as "Ansem's shadow", "The monster behind Ansem", "Terra's phantom", "Xehanort's shadowy other", and the "small, Darkside-like being" or "shadow", is a container for Terra's heart which is used by various vessels for Xehanort throughout the  series. While it contains Terra's heart and resembles a Heartless, it is not actually one of them, and Terra's heart frequently shifts between the One Behind and Terra-Xehanort's physical body.

It is first created by Terra-Xehanort to battle Aqua in Radiant Garden, and is inherited by his Heartless, Ansem, Seeker of Darkness. It is also channeled by Riku when he draws upon the darkness of Ansem still lingering in his heart during his battle with Roxas. The One Behind 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 One Behind 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 One Behind reappears and begins to fade, opening a portal to the Realm of Darkness which swallows it and Terra-Xehanort. 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 One Behind and attacks. The One Behind 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 the One Behind 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 One Behind. However, Riku is able to defeat the pair and 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 One Behind to grab Roxas and knock him out, so that he can bring Roxas to DiZ.

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 the One Behind. After a first battle, Ansem enlarges the One Behind and merges with it, but Riku defeats this form as well.

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

Kingdom Hearts III
The One Behind first appears in the Realm of Darkness, to fight off an unarmed Aqua, who is attempting to keep Ansem the Wise out of the Organization's clutches. After tossing Aqua across the End of Sea, the One Behind conjures a dark orb and launches it at Aqua's heart, transforming her into Anti-Aqua.

The One Behind next appears to defend Ansem when Hayner attempts to ambush him at Twilight Town. Ansem pursues but is waylaid by Vexen's Dusks, with the One Behind defending him again.

At 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 One Behind moves on its own to intervene and save them. It then breaks the chains binding the trio and lays the unconscious Aqua and Ventus safely on the ground. Terra-Xehanort is surprised by Terra's heart taking control of the vessel and insists that it had been consumed by the darkness, but the One Behind removes its gages and declares the promise to set things right it had made as Terra. While the One Behind is keeping Terra-Xehanort restrained, Sora quickly uses his Keyblade to expel Xehanort's heart from Terra's body, allowing Terra's heart to reclaim his rightful body and finally terminating the existence of the One Behind.

Design
The One Behind has an ink-black, muscular, humanoid form with a partially bandaged head that hosts relatively human features, bearing some resemblance to Master Xehanort. The bandages are gags representing the shackles Xehanort has placed on Terra's heart. 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.

When it merges with Ansem at The World That Never Was, the One Behind becomes much larger, and its lower body forms into an orb of darkness which surrounds Ansem, with two indigo wings extending from the sides and curving downward.

While part of the World of Chaos, the One Behind 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 One Behind 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.

Abilities
The One Behind has many dark powers with which it assists its host. It 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 One Behind 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 One Behind's most devastating ability allows it to sink into the ground, creating massive shockwaves every time it re-emerges. The One Behind 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 One Behind 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 One Behind 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 One Behind 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 One Behind 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 One Behind 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 One Behind 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 One Behind shoots back at it.

Ansem (second battle)
When Ansem and the One Behind face Sora on his own, their moveset will be identical to that of the first battle. However, the One Behind and Ansem can now use some new, devastating attacks. The first involves the One Behind and Ansem jetting across the battlefield, dealing damage as they collide with Sora. Either Guard or Aeroga can be used to halt the One Behind and Ansem. The second is, by far, the most damaging, the One Behind 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 One Behind, as well as the large burst of energy that results in the One Behind returning to Ansem.

Ansem (first battle)
During the first battle between Ansem, Riku, and the One Behind, Ansem will only shoot bursts of energy while the One Behind bombards Riku with punches. In Kingdom Hearts Re:Chain of Memories, however, the One Behind aids Ansem as he teaches Riku how to perform a Card Duel. The One Behind 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 One Behind, the gloves are off. While Ansem shoots bursts of energy, the One Behind 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 One Behind 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 One Behind 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 One Behind can latch onto Aqua, at this point rapidly press to free her. The One Behind 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 One Behind 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 One Behind by teaming up with Terra and shooting it with an immense amount of light energy.

Ansem (First Battle)
The fight is similar to the fight from the original game, as the guardian can toss blades of energy and be used as a battering ram by Ansem. However, it cannot briefly possess Riku or shoot out of the ground in rapid succession.

Unless Riku uses Nightmare's End to create a platform for him to stand on, the zero gravity of the area affords Ansem the advantage.

Ansem (Second Battle)
The fight transfers to solid ground for Riku, allowing him the use of Flowmotion to power up his attacks. However, Ansem hovers just beyond the area.

The guardian gains new attacks. Able to summon flames from the ground, throw several homing beams of energy, or simply punch the ground in rapid succession.

When dealt enough damage, the guardian manipulates the ground, pushing Riku all the way to the opposite end. Initally, it will simply send slow-moving dark orbs. These can be dodged by using Flowmotion on the walls until they stop flowing. When down to critical health, the guardian instead summons spheres of light, which send out powerful damaging beams at Riku. To stay ahead of this, its best to have at least three Curaga commands in stock.

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