Ansem, Seeker of Darkness

Xehanort's Heartless, as his name suggests, is the Heartless of Xehanort, and serves as the main antagonist of Kingdom Hearts and Reverse/Rebirth. While his identity as Xehanort is revealed in Kingdom Hearts II, he has taken his master's name, Ansem, and calls himself "Ansem, the Seeker of Darkness". In his human form he possesses a shadowy symbiote which he calls his "guardian".

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 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, Xehanort's Heartless continued on his path towards stealing many hearts.

Kingdom Hearts
Xehanort's Heartless, after ten years of being a Heartless, appears in a brown robe by the Destiny Islands Keyhole before the world's fall into darkness, insulting Sora's ignorance of what was to come, and completely ignoring Sora's questions.

The villain appears again to Riku, after his loss to Sora at Hollow Bastion. Shaken by his loss of control over the Keyblade to Sora, Riku lets himself be influenced by Xehanort's Heartless and his promise of power. Riku then lets his heart open up to the darkness, allowing Xehanort's Heartless to possess him.

Firmly in control of Riku's body, Xehanort's Heartless creates the artificial Dark Keyblade from six of the Princesses of Heart's hearts. This Keyblade possesses the power to unlock hearts, a property he soon tests on Maleficent, transforming her into a gigantic dragon. The dragon is felled by Sora and his friends, leading the Heartless to comment that the dark fairy was arrogant to believe that she manipulated the Heartless when it was actually the other way around. He insults Maleficent and takes his leave, waiting for Sora in the chamber of the Final Keyhole.

Sora arrives, finally reunited with Kairi, though she is comatose due to the loss of her heart. Xehanort's Heartless comments that she lacks a heart, and from his impersonal comments and unusual nature, Sora realizes that who he sees as Riku is not truly the friend he knew. Xehanort's Heartless introduces himself as "Ansem, the Seeker of Darkness". The Heartless goes on to explain that Kairi's heart is in Sora, and that his dark Keyblade is necessary to remove the imprisoned heart. Sora refuses to agree with the plan and they battle. Sora wins the duel, and Xehanort's Heartless retreats. However, his efforts are not in vain, as Sora stabs himself with the dark Keyblade, in order to free Kairi's heart as well as the six other Princesses. But this ends up opening the final Keyhole and the way to Kingdom Hearts, as well as the loss of Sora's heart and his brief transformation into a Shadow Heartless.

Xehanort's Heartless shortly returns, having obtained complete control over Riku's body and shaped it into one resembling his original human self. Now that the final Keyhole is open from Sora's sacrifice, he plans to kill Kairi now that her purpose in his plans has been fulfilled. Donald and Goofy bravely make a stand against Xehanort's Heartless in order to protect Kairi, but both know that, without Sora, they may not be able to stop him. Before he can dispose of the three, Riku projects himself from Xehanort's stolen body and manages to establish enough control to stop the Heartless in his tracks, allowing Donald and Goofy to escape with Kairi. This leads to an internal struggle between the two that Riku ultimately loses. His heart is forced out of his body, and Xehanort's Heartless completely takes control. Xehanort's Heartless then allows himself to be enveloped by the darkness flowing out of the Keyhole, and travels to the End of the World and Kingdom Hearts.

As Sora, his humanity having been restored by Kairi's light, arrives at the remnants of Destiny Islands at End of the World, Xehanort's Heartless appears, bearing Riku's form. He explains the fall of Riku to darkness, that he desperately sought to escape the island "prison" enough to open his heart to darkness. Afterwards, Xehanort's Heartless 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 Xehanort's Heartless. During the battle Ansem summons both his familiar, the Guardian Heartless, and a Darkside Heartless to assist him. As the battle rages on, Xehanort's Heartless leads the trio to the endless abyss where the Door to Darkness resides. This time, Xehanort's Heartless merges himself to the World of Chaos, a monstrous Heartless ship, and captures Donald and Goofy, though Sora rescues them.

After several more rounds of combat, Xehanort's Heartless, weakened, opens the Door to Darkness, imploring Kingdom Hearts to fill him with the power of darkness so that he can gain enough power to destroy Sora, Donald, and Goofy and shroud the universe in darkness. However, when Sora yells "You're wrong! I know now, without a doubt, that Kingdom Hearts is light!", Xehanort's Heartless is horrified to see light pour out of the door. The blinding light of Kingdom Hearts envelops and destroys Xehanort's Heartless, allowing Riku to take back his body.

Kingdom Hearts: Chain of Memories
However, because Riku served as the host for Xehanort's Heartless, a piece remains even after his destruction by the light of Kingdom Hearts. This bothers Riku greatly, who ends up in Castle Oblivion thanks to his heart resonating with Sora's, who is also in the castle.

Guided by a mysterious voice--who is actually DiZ--Riku climbs through the basements of Castle Oblivion, trying to rid himself of his darkness. However, Xehanort's Heartless appears early on, intending to overcome Riku, but fails. Still, Xehanort's Heartless remains, and even stirs the darkness in Riku's heart so that he may use it in battle, if he so chooses. The dark being remains confident that Riku will eventually give into the darkness.

At times, Riku almost becomes overwhelmed by the darkness, but King Mickey's light continually comes to the rescue, frustrating Xehanort's Heartless. Eventually, Riku overcomes his fear of the darkness, intending to use it as well as the light. He also additionally receives some guidance from the enigmatic DiZ, the true owner of the mysterious voice, who in turn leads Riku to Naminé. Naminé offers to seal away Riku's darkness and Xehanort's Heartless, but he instead chooses to face the Heartless on his own. DiZ then gives Riku a card that will give Xehanort's Heartless form, so that Riku can finish him off once and for all.

Xehanort's Heartless again tries to turn Riku to the darkness completely, and tries to defeat the young boy so that he can take over again, but is defeated. However, though he is defeated, Riku realizes that Xehanort's Heartless still lingers in his heart, and sets off with King Mickey in order to rid himself of the Seeker of Darkness forever.

Kingdom Hearts 358/2 Days
See also: Riku (Xehanort's Heartless)

Though Xemnas, the Nobody counterpart of Xehanort's Heartless, 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 Xehanort's Heartless, as well as manifesting his guardian, which pummels Roxas senseless before Riku can take control of his heart. In his new form, Riku mimics Xehanort before him and takes on the name Ansem.

Kingdom Hearts II
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 Xehanort's Heartless still needed to be stopped. He also reveals that the Nobody spawned at the same time as Xehanort's Heartless 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 guy.").

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 "Xehanort's Heartless", 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. What happens to Xehanort's Heartless is unknown, but is presumably disintegrated forever as the explosion's light fully restores Riku to the extent that he is robbed of his ability to open corridors of darkness.

Trivia

 * Xehanort's Heartless 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, though in human form it could possibly be an article of clothing. Notably, though, the similar outfit worn by Master Xehanort does not possess the emblem. 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 only.
 * The boots he wears are the same as those worn by members of the Organization XIII.

Notes and References
Sans-cœur de Xehanort