Ansem, Seeker of Darkness

Ansem, Seeker of Darkness serves as the main antagonist of Kingdom Hearts and the "Reverse/Rebirth" story in Kingdom Hearts: Chain of Memories. He is the Heartless of Xehanort, and retains the dark "guardian" which belonged to his original self.

Kingdom Hearts
''A researcher who studied the Heartless and delved into secrets of the worlds. He organized his findings into a single volume known as Ansem's report. When he inexplicably vanished, most of its pages were scattered to the winds.''
 * First entry

''As a researcher and ruler of his world, he studied the Heartless and delved into the many secrets of the worlds. While studying the heart and the darkness within it, he was possessed by darkness and ultimately destroyed his own world. He sacrificed his body to attain great power, and later possessed Riku's to regain a physical presence. Failure to stop him means the end of all worlds.''
 * Second entry

Kingdom Hearts: Chain Of Memories
Pursuer of darkness. To open the door to darkness, Ansem used the Heartless to steal countless hearts. He enslaved Riku, but met his demise at Sora's hand. Nonetheless, Ansem's shadow lives on in the depths of Riku's heart.
 * Reverse/Rebirth

Prior to Kingdom Hearts
In the pursuit of knowledge, Xehanort, at the time calling himself "Ansem", willingly gave his heart to darkness, creating both his Heartless and his Nobody, Xemnas. His Heartless was able to retain its selfhood for uncertain reasons, though it was believed by the true Ansem that this was due to Xehanort's willing abandonment of his heart.

He and legions of Heartless at his command continued his original self's pursuit of knowledge, believing it would be found in darkness, and stole countless hearts from both humans and worlds in search of this knowledge. But his ultimate goal was Kingdom Hearts, a supposed repository of tremendous power and knowledge. However, to access the heart of all worlds, he would need to open the Final Keyhole. For this, the Princesses of Heart were needed to complete the Keyhole. He also realized that the wielder of the legendary Keyblade needed to be found, for the Keyblade's ability to seal away the hearts of worlds from the preying Heartless could prove dangerous. So, believing that the Princesses and the Keyblade wielder are connected, he sent Kairi, a supposed resident of The Radiant Garden, Xehanort's home, as well as a Princess of Heart, to another world, a world where a future Keyblade wielder lived. As he waited for his theory to be proven, Ansem continued on his path towards stealing many hearts.

Ansem has a little penis

Kingdom Hearts: Chain of Memories
Here we see how ansems little penis is really really little and not even kairi will fuck him

Kingdom Hearts 358/2 Days
Though Xemnas, the Nobody counterpart of Ansem, soon becomes the main threat, the dark being still has a presence. In fact, the part of him that lingers in Riku starts to eat away at him, even forcing him to put on a blindfold over his eyes to keep him from "lying to himself".

This comes to a head when Riku travels to The World That Never Was at the behest of DiZ, with the intent to capture Roxas, the Nobody of Sora and a member of Organization XIII, so that the two can join and Sora can awaken from his slumber. However, Roxas proves to be too powerful for Riku to handle, so Riku is left with one option: Riku unleashes the darkness in himself, in order to gain the power necessary to defeat Roxas. However, this has the side effect of giving Riku the appearance of Ansem, as well as manifesting his guardian, which constricts Roxas in its fists and leaves him unconscious. In his new form, Riku mimics Xehanort before him and takes on the name Ansem.

Kingdom Hearts II
See also: Riku-Ansem

During the Battle of the 1000 Heartless at Hollow Bastion, King Mickey reveals the truth about "Ansem" to Sora, Donald, and Goofy; that he wasn't the real Ansem, but the Heartless of an impostor. While the trio are shocked and dismayed to learn this, Mickey assures them that Ansem still needed to be stopped. He also reveals that the Nobody spawned at the same time as Ansem is the leader of the Organization (as he had met the original Xehanort and been near his Nobody, it "kinda felt like bein' around the same fella.").

Riku remains like this for some time. His new appearance prompts him to refuse making contact with Sora and Kairi, despite him being able to easily do so. Still, when push comes to shove, Riku aids his friends, but never reveals himself as Riku (though Sora begins to suspect his mysterious helper's true identity), secretly leaving clues for Sora, Donald, and Goofy to find. But when Naminé chooses to use Riku's true name after he rescues her and Kairi from the Organization at The World That Never Was, things take a turn. Kairi, in her excitement, removes Riku's hood, only to see the face of the one who had endangered her a year ago. Still, she sees Riku underneath the dark form, and takes it in stride.

Riku is also forced to meet Sora when Kairi prevents him from taking his leave when Kairi and Sora reunite. At first, Sora does not realize it is Riku, refers to him as "Ansem", and informs him that he is grateful to him for Kairi's rescue while still enraged at the very thought of the evil deeds he had committed. Kairi helps him see through his altered form. They all then travel through the Nobody castle, and end up reuniting with King Mickey and DiZ, whose true name is Ansem the Wise. However, the machine that Ansem was using to encode Kingdom Hearts into data ends up exploding, allowing Riku to regain his true form. "Ansem" and his hold on Riku's heart are thought to be destroyed at this point, as the explosion's light fully restores Riku to the extent that he is robbed of his ability to open corridors of darkness.

Kingdom Hearts coded
Ansem appears in a memory from Jiminy's Journal in Destiny Islands, opening a Corridor of Darkness for Riku to Hollow Bastion.

Personality
Ansem is, thus far, the only Heartless with human-level intelligence. His knowledge of the worlds and hearts shows that Xehanort's keen intellect has survived the transition into a Heartless. However, what little shreds of humanity remained in the researcher have been obliterated. Ansem is cold, calculating and utterly ruthless. He treats people and their hearts as either tools to be manipulated and disposed of, or as simply cattle for his ravenous Heartless legions. He revers power and knowledge above all things, and despises ignorance.

The only times when his voice rises above a cold, monotonous drone is when he is berating Sora for his foolishness, or reveling in his latest triumph. However, the scientific genius of Ansem was blinded by obsession with the darkness. His arrogance ultimately led to his destruction when the door to Kingdom Hearts opened, and his greatest thesis was, in the end, proved wrong.

Appearance
Like his Nobody counterpart, Ansem's skin color is varyingly depicted as being tan in the game to quite dark in official artwork. He shares Xehanort's amber-orange eyes and long white hair, as well as the three spikes on the top and sides of his head, but he lacks Xehanort's bangs.

He wears silver-white gloves on his hands, each with a grey band on the wrist, while the rest of the glove, which has three black buttons lining one side, bells outwards. There is a black paulron on both of his shoulders, each attatched to his coat by two straps. He also wears black pants and black, silver-lined, knee-high boots identical to the ones worn by Organization XIII members. On the Heartless' chest is the Heartless emblem, but it is unclear if this is simply an article of clothing or actually part of him. It seems that it is not part of him, as when Ansem creates the World of Chaos, he loses both of his jackets and his gloves, and the emblem transfers to the World of Chaos. He also grows to a tremendous size and his eyes become completely orange in this form. It is possible, though, that the World of Chaos is simply an extension of Ansem himself.

"Ansem's" black leather coat is red on the inside and lined with yellow, and also has a large grey section that covers the lower half and shoulders of the coat. Upon closer inspection, this grey section seems to be attatched by several small pins or buttons around the hip and neck sections of the coat, as well as by a belt that wraps around the Heartless's back and attatches to the rim of the coat. This coat is high-collared and completely unzipped, showing the second, shorter jacket the Heartless wears underneath. Ansem's second jacket is white and high-collared much like the outer coat. It is mostly unbuttoned and exposes "Ansem's" chest before being closed up be three buttons and two belts near the Heartless's waist, only to flare outward again, leaving three buttons undone.

Weapons
Ansem uses no single exclusive weapon.

When possessing Riku, taking on his first seen physical form, he uses the Keyblade of People's Hearts created from the six captured hearts of the Princesses of Heart. However, he can only use this weapon while possessing the body of a Keyblade Master. Later, in his World of Chaos form, he wields a weapon as massive as he is, created from two Soul Eaters combined at the handle, facing opposite directions.

When fought on his own, he simply uses the power of darkness, and his Guardian Heartless, using no physical weapon. When Riku takes on his form, he uses his Soul Eater and Way to the Dawn, just as he does normally.

Quotes

 * "This world has been connected, tied to the darkness, soon to be completely eclipsed."
 * "So, you have awakened at last, princess. The keyhole is now complete. You have served your purpose, but now it's over."
 * "There is so very much to learn. You understand so little. A meaningless effort. One who knows nothing can understand nothing."
 * "Take a look at this tiny place. To the heart seeking freedom, this island is a prison surrounded by water. And so this boy sought out to escape from his prison. He sought a way to cross over into other worlds and he opened his heart to darkness."
 * "Don't bother. Your voice can no longer reach him where he is. His heart belongs again to darkness."
 * "All worlds begin in darkness and all so end. The heart is no different. Darkness sprouts within it. It grows, consumes it. Such is its nature."
 * "In the end, every heart returns to the darkness whence it came. You see, darkness is the heart's true essence."
 * "So, you have come this far and still you understand nothing. Every light must fade, every heart return to darkness!"
 * "Behold the endless abyss! Within it lies the heart of all worlds: Kingdom Hearts! Look as hard as you are able, you'll not find even the smallest glimmer of light. From those dark depths are all hearts born, even yours."
 * "Darkness conquers all worlds!"

Trivia

 * Ansem appears to have characteristics of both Emblem and Pureblood Heartless, in both his human and world forms. In both forms, he carries the Heartless Emblem on his chest, the distinguishing mark of an Emblem Heartless. Though this could possibly be an article of clothing for his human form, the emblem is notably absent from Master Xehanort version of the same outfit. Furthermore, the shadowy symbiote attached to him in both forms possesses the "hollow heart" shared by many other Pureblood Heartless. Though it is uncertain what the situation of this Heartless is, it is possible that this makes him both an Emblem and Pureblood Heartless.
 * Though their characters never interacted, Akio Ohtsuka was castmates with his real-life father and fellow seiyuu Chikao Ohtsuka (Captain Hook) in the first game. In Kingdom Hearts Birth by Sleep, Chikao Ohtsuka even voices the metaphysical "father" of Ansem, Master Xehanort.
 * The boots he wears are the same as those worn by male members of the Organization XIII (excluding Roxas), as well as Ansem the Wise's apprentices (save for Ienzo).

Notes and References
🇰🇲 Sans-cœur de Xehanort