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. Xehanort's Heartless referred to himself exclusively as Ansem from a uncertain point on, taking his name from Ansem the Wise, his original self's teacher. The Heartless continued his original self's pursuit of knowledge, believing it would be found in darkness. To this goal, he sought Kingdom Hearts. 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.

Following the Keyblade's trail
Xehanort's 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. Influenced by Xehanort's Heartless' words, Riku open his heart to darkness, and Xehanort's corrupted heart.

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 beleive that she manipulated the Heartless when it was actually the other way around. he took his leave, waiting for Sora in the chamber of the final Keyhole.

Sora arrives, finally reunited with Kairi's lifeless body. Xehanort's Heartless commented that she lacked a heart, from which Sora realizes that what he sees as Riku is not truly the friend he knew, to which Xehanort's Heartless introduces himself as "Ansem, the seeker of darkness". The Heartless goes on to explain that Kairi's heart was in Sora, and that his Keyblade is necessary to remove the imprisoned heart. Sora refuses to agree with the plan and they battle. Sora wins the duel, and Riku's boy fades away.

Xehanort's Heartless shortly returns, in a from similar to his original self, satisfied that the final Keyhole is open from Sora's sacrifce, 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 and the others to escape. Xehanort's Heartless then allowed himself to be enveloped by the darkness, travelling to End of the World.

End of the World
As Sora arrives at the remnants of Destiny Islands at End of the World, Xehanort appears, bearing Riku's form. He explained 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 was the heart's true essence. Sora denies this, in words and battle with Xehanorts Heartless and his Guardian. After taking enough damage, the Heartless retreats, allowing a Darkside to combat Sora. Xehanort's Heartless' presence is forced when Sora defeats the giant Heartless. After further damage, the Heartless draws Sora and his frineds into the "endless abyss" where Kingdom Hearts is found. Xehanort's Heartless' form this time around is the formiable World of Chaos, a monster ship of many colors. After the solitary Sora incurs enough damage on Xehanort's Heartless, he forms a impenetrable shield in the form of a Heartless emblem, forcing Sora to rescue his friends from the darkness and destroy portions of World of Chaos. This causes the shield to break and Sora finally defeats the Heartless with his friends.

After the last tiring battle, Xehanort's Heartless calls upon Kingdom Hearts to give him power, opening the doors and allowing darkness to escape. The request falls on deaf ears when light also bursts through the doors, illuminating the Heartless and killing him as he wondered why light was in Kingdom Hearts.

Haunting Riku
Because Riku served as the host for Xehanort's Heartless, a piece remained even after his destruction by the light in Kingdom Hearts.