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.
After conversing with Sora over the true nature of the heart, Ansem, Seeker of Darkness proceeds to summon his original persona's guardian, preparing to battle Sora due to the Keybearer's naïvety. The guardian later appears alongside Ansem during his second battle with Sora, again transforming and increasing in size to become part of the World of Chaos. The guardian, along with the rest of the World of Chaos, is destroyed at the hands of Sora, Donald, and Goofy after the climactic battle.
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 take control, growing 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.
The Guardian has an ink-black, muscular, humanoid form with a partially bandaged head that hosts relatively human features, baring 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. It lastly 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 massive wings, 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.
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 beat its massive wing-like arms 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.