Ansem

Ansem, Seeker of Darkness is the primary antagonist of the first Kingdom Hearts game. Though he appears human, he is actually the Heartless born from Master Xehanort, the sinister Keyblade Master and main antagonist of Kingdom Hearts: Birth by Sleep. Due to him having intentionally separated his heart from his body and soul before he gave into the darkness, Ansem retained his self-hood, and intelligence after his transformation into a Heartless. Because he could think out his plans and pass for a human, he was able to destroy nearly every world in the Kingdom Hearts universe without revealing his identity, up until he confronted Sora at the Final Keyhole in Hollow Bastion. Due to his unique "birth", Xehanort's Heartless appeared to have characteristics of both the Emblem and Pureblood Heartless, though it is unclear exactly which category he falls under.

Birth By Sleep
The Seeker of Darkness was brought into being by Xehanort, whose heart had possessed the body of Terra and had assumed the name of the king of Radiant Garden, Ansem the Wise. Terra-Xehanort willingly abandoned his heart in the pursuit of knowledge and power, splitting his being into the Heartless Ansem and the Nobody Xemnas. Since Terra-Xehanort had willingly cast his heart into darkness, it is believed that this is the reason that Ansem retained his memories and sense of self unlike other Heartless.

The self-styled "Seeker of Darkness" and the legions of Heartless at his command continued Xehanort's pursuit of knowledge in the belief that it lay with darkness, and stole countless hearts from both humans and worlds in their search for their ultimate goal, a legendary repository of tremendous power and knowledge known as Kingdom Hearts. To reach this heart of all worlds, Ansem would need to open the Final Keyhole, and to do that, he would need the Princesses of Heart to complete it first. He also realized that he needed to locate and neutralize the wielder of the legendary Keyblade, to prevent them from saving the worlds from his Heartless. He detected a power within Kairi, a Princess of Heart living within Radiant Garden, linking her to a guardian of light, which he misidentified as an innate connection between Princesses and Keyblade wielders. In truth, the power within Kairi was the charm Aqua had placed upon her, with much the same effect—when he cast her loose in the Ocean Between, she was naturally drawn to Sora and Riku on the Destiny Islands. Meanwhile, Ansem traveled through time to Destiny Islands and completed a task set before him by his original persona: to pass on his time-travelling ability to Xehanort's younger self and set him on the path to establishing the Real Organization XIII and obtaining the legendary χ-blade.

Kingdom Hearts
For nine years, Ansem traveled across the worlds, having no physical presence but manipulating the Heartless as they overwhelmed several worlds. He would return to the Destiny Islands shortly before the islands were also cast into darkness, mocking Sora's ignorance of what was to come. Later, Ansem would appear before Riku in Hollow Bastion. While in service to Maleficent and her plan to unlock the door to darkness with the Princesses of Heart, Riku had fought against Sora and lost. Ansem encouraged Riku to allow the darkness into his heart, to accept its power to become stronger. When Riku allowed himself to be overtaken by darkness, Ansem took possession of his body, once more gaining a physical presence.

While in possession of Riku, Ansem used the hearts of the captive Princesses to fashion the Keyblade of Heart. This Keyblade has the power to unlock people's hearts and release the darkness within. Ansem uses it first on Maleficent, unlocking the full extent of her dark power and transforming her into a dragon. Leaving Maleficent to battle Sora, Donald and Goofy, Ansem attends to the Final Keyhole. However, he cannot complete it without the heart of Kairi, the last Princess of Heart.

After Sora and his friends have defeated Maleficent, they confront Riku in the castle's Grand Hall. Ansem reveals himself to Sora and also reveals that Kairi's heart rests within Sora. He attempts to strike Sora down with his dark Keyblade, but Sora resists and the two duel alone without Goofy and Donald interfering due to a magical barrier. Sora wins the duel and Ansem retreats, but his intention of opening the Final Keyhole is yet fulfilled due to Sora intentionally stabbing himself with Ansem's Keyblade in order to release Kairi's heart. This action restores Kairi, but at the same time the Keyhole is opened and Sora is transformed into a Heartless.

Regaining his strength, Ansem casts aside Riku's heart and assumes a form similar to his original human guise. He reappears before Kairi, Donald and Goofy and intends to kill them now that Kairi has served her purpose, but Riku briefly calls out to the heroes from within Ansem and tells them to run away as the Heartless are coming. Later, Sora has been restored to his human form by Kairi and confronts Ansem at the End of the World. Upon the broken remnants of the Destiny Islands, Ansem explains how Riku fell into darkness because of his desperate desire to leave the islands and travel to other worlds. Afterward, Ansem explains his philosophy: that worlds and hearts begin and end in darkness. Thus, darkness is the heart's true essence. Sora denies this, in words and in battle, with Ansem. During the battle Ansem summons both his familiar, the Guardian, and a Darkside Heartless to assist him. As the battle rages on, Ansem leads the trio to the endless abyss where the Door to Darkness resides. This time, Ansem merges himself to the World of Chaos, a monstrous Heartless ship, and captures Donald and Goofy, though Sora rescues them.

The battle is long and hard, but Sora, Donald and Goofy are eventually victorious against Ansem. Though in a weakened state, Ansem is able to open the Door to Darkness, revealing Kingdom Hearts. He calls out to Kingdom Hearts, imploring it to fill him with pure darkness and envelop the universe, but to his horror he finds that Kingdom Hearts represents light. Brilliant light floods out of the door, enveloping Ansem and destroying him, allowing Riku to reclaim his body.

Chain of Memories
Though seemingly destroyed by the light of Kingdom Hearts, Ansem survives as a fragment of darkness within Riku's heart. Riku discovers this while exploring Castle Oblivion, his heart resonating with Sora's. Upset and guilt-ridden by his past actions, Riku unintentionally allows Ansem to resurface. The Seeker of Darkness taunts the boy and encourages him to surrender his heart and his body once again, but his efforts to manipulate Riku are constantly interrupted by King Mickey whose power of light supports and protects the boy. Riku and Mickey are later assisted by the mysterious individual DiZ, who leads them to a young girl called Naminé. Possessing the ability to alter people's memories, Naminé offers Riku a means of ridding himself of Ansem forever by sealing the darkness inside of him. However, Riku decides that he must confront the darkness inside him on his own and make its power his own without turning away from the light. DiZ supports Riku by offering him a card that will give Ansem form, allowing Riku to confront him. Ansem again attempts to turn Riku to the darkness completely, and tries to defeat the young boy so that he can take possession of his body once more. However, though he is defeated, Riku realizes that Ansem still lingers in his heart, and sets off with King Mickey in order to rid himself of the Seeker of Darkness forever.

358/2 Days
Within Riku's heart, Ansem's darkness starts to eat away at him, forcing the boy to wear a blindfold to put a stop to Ansem's false visions. This comes to a head when DiZ sends Riku to capture Sora's Nobody, Roxas, so that the two can join and Sora can awaken from his slumber. Riku catches up to Roxas at The World That Never Was, but Roxas proves to be too powerful and Riku is left with only one way to overwhelm the Nobody: unleashing his inner darkness. This allows Riku to manifest Xehanort's powerful guardian, who seizes Roxas and knocks him unconscious, but also forces Riku to take on Ansem's appearance. Feeling that he has sacrificed his identity, Riku takes on Ansem's name.

Kingdom Hearts II
While Riku had assumed Ansem's form, he began experiencing dark visions imposed upon him by the Seeker of Darkness' remnant lingering in his heart. To stop these visions, Riku began wearing a blindfold.

Working together with DiZ, Riku managed to capture Roxas so that DiZ could merge him with Sora, who slumbered within the white room beneath an old mansion in Twilight Town.

During the Battle of the 1000 Heartless at Hollow Bastion, King Mickey reveals the truth about "Ansem" to Sora, Donald, and Goofy—the Heartless wasn't the real king of Radiant Garden, but the Heartless of an impostor. While the trio are shocked and dismayed to learn this, Mickey pacifies them by telling them that Ansem was still a powerful enemy that needed to be stopped.

Following the destruction of the Kingdom Hearts Encoder Ansem the Wise was using, the resulting explosion of light restores Riku to his normal form, implying that "Ansem" has finally been destroyed and his hold on Riku broken once and for all.

Dream Drop Distance
Due to Young Xehanort's travels through time, "Ansem" is among the versions of Xehanort summoned and made a member of the True Organization XIII. "Ansem" taunts Riku throughout his journey through the Sleeping Worlds until Riku defeats him and escapes into the real world to thwart the real Organization XIII. Likewise he was forced to retreat with the rest of the Real Organization when Lea, Donald and Goofy stall them to prevent Sora from becoming their thirteenth member.

Kingdom Hearts III
Ansem first appears in Twilight Town alongside Xemnas, with the two taunting Sora and encouraging him to free Roxas before summoning several Heartless and Nobodies as a distraction. After this, they watch Sora in the town, where Xigbar berates them for directly guiding Sora.

Later, Ansem visits the Dark World to confront Ansem the Wise about Subject X, a girl Terra-Xehanort had admired as an apprentice. Though Aqua tries to stop him, Xehanort's Heartless corrupts her with darkness before leaving with DiZ turning her into a cold and ruthless Anti-Aqua. Ansem takes DiZ to the Old Mansion and taunts him with the knowledge of the children used in the experiments, but Olette rescues DiZ with the help of Hayner, Pence the former two who distract him from noticing Olette taking Diz away, and Vexen who commands a group of Dusks to attack Ansem. With Hayner going as far as to attack the dark being, here he also finds out thet Vexen has defected from the Organization when he figures out that Vexen was the one who sent the Dusks to come after him. Ansem joins the real Organization XIII at the Keyblade Graveyard for the final clash between light and darkness. He fights Sora, Riku, and Mickey alongside Young Xehanort and Xemnas with help from Master Xehanort, but is ultimately slain in battle. He reflects on his journey that he and Riku had together, Riku admits to Ansem that he is going to miss him and Ansem congratulates Riku for becoming a powerful Keyblade master, admitting to him he never stood a chance against him.

He also admits to Sora and Riku that part of him wanted to defy his fate but once he discovered that many members defected from the Organization he found that he cared no longer. Sora attempts to comfort Ansem despite his actions against him and Riku but Ansem tells the trio that it is time for them to move on and encourages them to go forth and continue before fading away.

Personality
Ansem, unlike most other Heartless, still possesses human intelligence and memory. He is significantly intellectual and speaks in a low, cold baritone that reflects his mindset. He holds vast knowledge of the human heart and of the power of darkness, but views the hearts of others as mere tools to manipulate and control in pursuit of his ambitions.

Completely obsessed with the darkness and unlocking its full power, Ansem seeks to acquire Kingdom Hearts and unleash absolute darkness upon the universe, destroying all worlds. His arrogance ultimately leads to his destruction when the door to Kingdom Hearts is opened, and his greatest thesis is, in the end, proved wrong.

Abilities

 * Power of Darkness - Ansem can wield darkness as he sees fit, usually using it to teleport, conjure barriers, generate destructive shockwaves and summon Heartless.
 * Possession - Ansem has no body of his own, but is capable of taking control of those whose hearts are steeped in darkness, as he does with Riku.
 * Swordplay - While possessing Riku, Ansem demonstrates a proficiency in swordfighting with his demonic blade, the Soul Eater.
 * Time Travel - In his non-corporeal state, Ansem can shift between the past, present and future.