Ansem, Seeker of Darkness

Xehanort's Heartless is the main antagonist of Kingdom Hearts, though at that time he called himself Ansem, the Seeker of Darkness, and it wasn't revealed until Kingdom Hearts II who he really was.

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 reports. When he inexplicably vanished, most of its pages were scattered to the winds.''

Kingdom Hearts Re: Chain Of Memories
Sora's Story

Riku's Story

Kingdom Hearts II
''Mysterious wielder of darkness.

What could his true purpose be?''

Separation
Xehanort's Heartless was spawned from the heart of Xehanort, or Ansem as he called himself, when he gave his heart to darkness in the pursuit of knowledge (this also gave birth to the leader of Organization XIII, Xemnas). However, he had been calling himself Ansem, taking his name from Ansem the Wise, his original self's teacher, for some time, and he continued this practice even as a Heartless. However, unlike others who became Heartless, "Ansem" kept the memories of his human life for reasons not even he knows.

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, 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.

Following the Keyblade's Trail
Xehanort's Heartless, after nine 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 they may not be able to stop him without Sora. 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.

End of the World
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.

Haunting Riku
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, 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.

New Developments
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 and the darkness that he lives in prove to be too much, 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, who used the Guardian to knock Roxas out. So, in his new form, Riku takes on a new name: Ansem.

During the Battle of the 1000 Heartless at Hollow Bastion, 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 dismayed to learn this, Mickey assures them that Xehanort's Heartless still needed to be stopped.

Riku remains like this for some time, though he somehow gains a Keyblade called the Way to the Dawn at a certain point. 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. 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.

In Battle
Xehanort's Heartless body is actually Riku's body. When he fully gained control of it he changed the body's appearance into a body similar to how he looked when he was human. His familiar Heartless, the Guardian, is the one that attacks. He could ram into people with his Guardian, using dark energy to cause more damage. His Guardian also shot out crescent shaped dark disks at the opponent. Xehanort would also use his Guardian to grab people, and slowly drain their health. During this time the Guardian will control the body at a range by making it create a force field of darkness around itself. Xehanort's best move was a replica of Riku's Dark Aura attack; first the Guardian separates itself from the body and then disappear into the ground, next it erupts from the ground several times and follows Sora across the arena, it then finishes with the body causing columns of darkness from to erupting from the ground. As the World of Chaos, Xehanort's Heartless used a more direct style of fighting, attacking with a double bladed Soul Eater polearm. When Riku took on the appearance of Xehanort's Heartless he still fought the same, but during the face off against Roxas he was temporarily overtaken with darkness. He summoned the Guardian Heartless, which proceeded to pummel Roxas senseless and knock him unconscious before Riku could stabilize his heart again.

Card
see:Ansem (Enemy Card)

Dark Riku (Xehanort's Heartless)

 * "Come."
 * "Is that it?"
 * "Kneel!"
 * "Now witness true Power!"
 * "Welcome Oblivion."
 * "Here it comes!"
 * "Behold the power of Darkness!"
 * "Open your heart to Darkness!"
 * "Surrender!"
 * "It's over!"
 * "Well, what now."
 * "Foolish boy."
 * "You are Nothing."
 * "My force...has failed me..."

Xehanort's Heartless

 * "Power!"
 * "Come Guardian!"
 * "Insolent."
 * "Submit!"
 * "Come, open your heart!"
 * "My strength returns."
 * "Witness this!"
 * "What do you hope to accomplish?"
 * "Do not deny who you are."
 * "The Final Darkness is now!"
 * "Take this!"
 * "Why do...you refuse the...darkness?"

Trivia

 * In the North American version, Xehanort's Heartless was voiced by Billy Zane, whose only Disney role is John Rolfe in Pocahontas 2. Much like his previous self, Xehanort's Heartless was voiced by popular seiyuu Akio Ohtsuka in the Japanese version, who also voiced Judge Magister Gabranth of Final Fantasy XII. However, Billy Zane did not reprise his role in Kingdom Hearts Re:Chain of Memories and Kingdom Hearts II; instead, Richard Epcar provided the voices as well as dubbed over the former's voice during Roxas's flashback of Sora's memories.
 * 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.
 * It is believed that the reason Ansem still retained his humanity after becoming a Heartless was by doing so of his own will, in addition to having a strong heart. This can be seen in Sora, who gave his heart willingly to return Kairi's from his body with the dark Keyblade. Shortly after, he returned as a Shadow (probably because he did not possess as much darkness as Xehanort's Heart) and found his way to Kairi, who restored Sora to his original form. However, Axel and Larxene state in Kingdom Hearts: Chain of Memories that Sora still retained his own mind as a Heartless, presumably due to the strength of his own heart and the Keyblade's power,
 * In Kingdom Hearts Chain of Memories, Sora can get the Ansem enemy card, even though he's not a boss for Sora. Sora can get it on the thirteenth floor after the player beats Riku's story.
 * It is unconfirmed whether or not Xehanort's Heartless is a Pureblood or Emblem Heartless due to the emblem on his chest, but it is speculated that he is most likely a Pureblood and that the emblem on his chest is just to prove he became a Heartless.
 * In Reverse/Rebirth (Riku's story in the English version of Kingdom Hearts: Re:Chain of Memories), when Riku undergoes the sleight tutorial against Xehanort's Heartless (then under the name Ansem), there is a typing error in one of the messages.