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Kingdom Hearts I
Like his video game counterpart, Ansem is split from Xehanort, leaving Xemnas behind as his Nobody. As Xehanort, he banished his mentor, Ansem the Wise, to the Realm of Darkness when the latter told him to discontinue his research, and then he stole his mentor’s research, making it his own. Then when he became Ansem, he releases Heartless all over his world, ruining it, and making it known as Hollow Bastion. He took a young Kairi to another world, knowing that her heart will be drawn to a Keyblade wielder. Then he struck down his coconspirators, turning them into Heartless, while their bodies are turned into Nobodies.
Now a formless Heartless in a black cloak, Ansem has been lurking around the castle, coercing Maleficent to capture the Princesses of Heart so their hearts will combine to open the keyhole to darkness, with the promise of giving Maleficent enough power to rule the world. In reality, his true intentions were to bide his time until he gets a new physical form, kill Maleficent when she is no longer of any use to him, and enter the final keyhole to go through the Door to Darkness and unleash Heartless to consume the Heart of All Worlds. The plot plays out as it does in the original game with stark contrasts.
When Ansem finally emerges at Hollow Bastion, instead of convincing Riku to reach down into the fullest darkness so he can possess him, he psychologically tortures him to the point where he takes his body by force, with no consent whatsoever. Now in physical form, Ansem approaches Maleficent at the Grand Hall, and and shows his new Keyblade, made from the extracted hearts of the six princesses. He says that the keyhole is opened, the Heartless will overrun the world, while Maleficent says that it doesn’t concern her, for she holds power over darkness rather than the other way around. Ansem’s response was, "Oh really? Shall we test that?" and stabs Maleficent in the heart with his Keyblade. Unlike the game, Maleficent does not transform into her dragon form when he "unlocks" the darkness within her. Instead, the darkness consumes and kills her as Ansem smiles as if he anticipated this result.
When Sora, Donald, and Goofy arrive at the scene, Ansem reveals himself as Ansem, the Seeker of Darkness, making them believe that he’s the original Ansem. He states that Kairi’s heart rests inside Sora’s heart. Instead of battling them, Ansem convinces Sora to stab himself in the heart with his Keyblade to set Kairi’s heart free. Ansem vanishes, and Sora uses it on himself, and disappears, re-emerging as a Heartless, as Ansem knew that this would happen. As he told Maleficent earlier, a flood of Darkness and Heartless will emerge from the Final Keyhole once it's been completed by the release of all Seven Princesses' hearts, which could kill Donald, Goofy, and all the Princesses (Kairi included) anyway.
In fact, once Sora's body has vanished and his heart is falling into Darkness, and Kairi has come to, Kairi refuses to accept what's befallen Sora and stands defiant, prompting Ansem to re-appear, having now re-shaped Riku's body into a new form that resembles Ansem's former human self, and declare that since Kairi's purpose to his plan has been served, he will kill her personally. Unlike in the game when Riku’s heart tries to block Ansem, he mocks him for his efforts, and banishes his heart to darkness. Ansem then looks at the oncoming darkness that threatens to tear apart Hollow Bastion with joy, remarking that it's time for all to return to darkness. The Beast sees Ansem entering the keyhole, while he glanced back daring anyone to try to stop him.
At the End of the World, when Sora, Donald, and Goofy confront Ansem, Ansem mocks Sora for worrying for the fate of Riku's heart while also mocking Riku for opening his heart to darkness without knowing that it would mean his heart would eventually return to darkness. He explains that he needed the seven Princesses captured by Maleficent and the Council of Disney Villains, because only the Princesses pure hearts of light can create the way to the Door to Darkness, behind which the Heart of All Worlds, Kingdom Hearts, can be found. Kingdom Hearts has been consummated behind the Door to Darkness, which has been materialized in the core of the Heartless' world, both as result of the Heartless devouring countless worlds' hearts. His plan is to plunge the World into eternal darkness, and he tears apart Destiny Islands to reveal the core of the Heartless, and turns into the World of Chaos like in the video game. Just like in the game, Ansem calls upon Kingdom Hearts to fill him with the power of darkness, but Sora says that Kingdom Hearts is light. To his horror, he was engulfed and destroyed by the light. With Ansem gone, Riku's body is restored to him within the Realm of Darkness.
Chain of Memories
Ansem re-emerges as a specter of darkness that lingers in Riku’s heart. Just like in the game, Ansem tempts Riku to go back into darkness. When Lexaeus was killed, Ansem invokes emotions of fear, anger, sorrow, doubt, and regret in Riku because those emotions will strengthen him enough to eventually overtake Riku and gain a physical existence once more. In the final battle with Riku, Ansem calls forth his guardian in an attempt to brutally strangle Riku to the point where his body and heart will succumb so he can take his body again. Riku lands the killing blow, and Ansem declares that he will return, and so long as Riku's darkness is ruled by his dark thoughts and emotions, and as Riku continues to think of him, then he shall forever linger in his heart and never go away.
Eventually, Ansem has been purged completely from Riku’s body, but because Riku had to fully immerse himself in his dark power, which allows him to take his powers and even his appearance.