Ansem, Seeker of Darkness

Xehanort's Heartless is the main antagonist of Kingdom Hearts.

Separation
Xehanort's Heartless was spawned from the heart of Xehanort 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.

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 other worlds. As he waited for his 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.

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 take over.

Firmly in control of Riku's body, Xehanort's Heartless creates the 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 takes his leave, waiting for Sora in the chamber of the final Keyhole.

Sora arrives, finally reunited with Kairi's lifeless body. 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.

Xehanort's Heartless shortly returns, in a form similar to his original self, satisfied that the final Keyhole is open from Sora's sacrifice, but not willing to allow the revived Kairi to live. Before he can do so, Riku establishes enough control to stop the Heartless in his tracks, allowing Kairi, Donald, and Goofy to escape, but Riku's heart ends up being forced out of his body, giving Xehanort's Heartless complete control. Xehanort's Heartless then allows himself to be enveloped by the darkness flowing out of the Keyhole, and travels to End of the World and Kingdom Hearts.

End of the World
As Sora 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 battle with Xehanort's Heartless and his Guardian, as well as a Darkside Heartless. 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 also uses the World of Chaos, a monstrous Heartless ship.

After several more rounds, Xehanort's Heartless, weakened, opens up the Door to Darkness, imploring Kingdom Hearts to fill him with the power of darkness. However, something pours out of the door that Xehanort's Heartless never expected: light. 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, 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 counter part 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 anyone from figuring out it out.

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. In his and Roxas's second encounter, Riku unleashes the darkness in him, 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. So, in his new form, Riku takes on a new name: Ansem.

Riku remains like this for some time, though he somehow gains a Keyblade called the Way to the Dawn at a certain point. This 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 his identity (though Sora begins to suspect so). 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. Sora also sees Riku's true form underneath his dark 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.