In Kingdom HeartsEdit
As a scientist, Ansem is incredibly gifted with great technology that made Radiant Garden the special place it was. Though capable of going into potentially dangerous fields of study, he has the common sense to know when enough is enough. When he is betrayed by his apprentice Xehanort, he becomes consumed with vengeance and hate, which allows him to survive the Realm of Nothingness and make his way out under the guise of DiZ, having become ashamed of his good name being tarnished. His determination to make Xehanort pay for his misdeeds lead to him seeing allies as pawns between viewing Namine as expendable for being a Nobody and thinking of Sora as a weapon against the Organization. By the climax of the game, he repents for his mistreatment of his allies and within the Realm of Darkness, has spent much of his time reflecting on what he had done.
Kingdom Hearts Birth by SleepEdit
In the aftermath of the battle between Master Eraqus's pupils and Master Xehanort, Braig led Ansem and Dilan to an amnesiac young man who introduced himself as "Xehanort". Ansem took the young man in, as well as the Keyblade and armor laying next to him, and worked to help him regain his lost memories.
Ansem is later seen walking through the halls of his castle with Ienzo, while both eat sea-salt ice cream. Ienzo suddenly stops, causing Ansem to look behind himself and see Xehanort and Braig in another hallway, deep in conversation.
Between Kingdom Hearts Birth by Sleep and Kingdom Hearts: Chain of MemoriesEdit
As the years went on, Ansem the Wise became worried that the darkness of people's hearts would one day swallow his beloved home. In order to understand and counter it, Ansem began conducting tests on Xehanort's heart. Ansem also hoped that this would unlock Xehanort's hidden memories, and his apprentice showed interest in the plan. However, the more he probed, the more he became worried about Xehanort's well-being and his possibly superhuman abilities.
Still, Ansem eventually constructed a large laboratory in the basement of his castle, encouraged by Ienzo. However, Ansem's six apprentices used the laboratory in order to conduct inhumane experiments on the human heart and the darkness within it. Ansem immediately ordered his apprentices to cease their experiments and sealed the chamber. Still, Ansem sank into despair over his error in experimenting on Xehanort. Gummi Ship travel was now possible, thanks to Xehanort opening the door in Ansem's basement, and a ruler of another world, Mickey Mouse, soon arrived. The two rulers became great friends, conversing over many subjects, and their companionship began to heal Ansem's heart. King Mickey eventually advised Ansem the Wise to review the research data; Ansem discovered the Ansem Reports, penned under his name, but written by Xehanort. Ansem began to see that the damage he had done had become irreversible.
Eventually and unfortunately, the six apprentices dabbled too deep into the darkness behind his back. They were consumed by it, with Xehanort producing his Heartless and Nobody, while the other five apprentices also produced powerful Nobodies, making the elder members of Organization XIII. His mistakes had caught up to him when Ansem was plunged into the Realm of Nothingness by his apprentices, and became embittered at their betrayal.
For some time, Ansem remained in the empty realm, constantly writing his own reports in order to maintain his sanity. Here, he also discarded his old, stolen name, and became DiZ ("Darkness in Zero"). Eventually, he found a way to escape when he gained the power of darkness, though he did not intend to let himself be consumed by it like his apprentices. With his new found powers, DiZ also garbed himself in a red robe and a mask made of red bandages in order to fully conceal his real identity.
Using the Corridors of Darkness, DiZ found Twilight Town, and took refuge in an abandoned mansion on the outskirts of the town. There, he began researching the Nobodies and the Heartless, as well as watching Sora and King Mickey's efforts to defeat Ansem, Seeker of Darkness, from the sidelines.
Kingdom Hearts: Chain of MemoriesEdit
DiZ, hoping to gain revenge on the Organization, infiltrates Organization XIII's second base, Castle Oblivion. There, he monitors Sora and encounters Naminé. DiZ magically speaks to Riku, waking him from his rest between realms, and leading him through the castle to assist Riku in his quest to defeat his darkness. He appears to Riku at the second basement, using his powers to disguise himself as the Heartless "Ansem, Seeker of Darkness", but Riku sees through the disguise, and DiZ reveals himself as the "Voice" that had been leading Riku. With Riku is King Mickey, and DiZ is privately saddened that they cannot talk as they used to, but Mickey begins to suspect DiZ's identity. DiZ provides Riku and the King with the black coats of the Organization, as well as a card that will draw out "Ansem". Using the card, Riku defeats the Heartless, though a part of him still lingers in Riku's heart. Riku and King Mickey then set off, and DiZ appears to them at the crossroads in the grassy plain, asking which path they will take. Riku chooses the middle road, which DiZ refers to as the "twilight road to nightfall". Riku corrects him, calling it the "road to dawn".
Kingdom Hearts 358/2 DaysEdit
DiZ soon returns to Naminé to aid her in the task to restore Sora's memories, after they were broken apart in Castle Oblivion. Since the castle is still under control of Organization XIII, DiZ convinces Naminé to move Sora, Donald, and Goofy to his lab in Twilight Town, in order to keep them safe. However, Sora is taking too long to awaken, and DiZ convinces Riku to retrieve Roxas, the Nobody of Sora. Riku has no choice but to take on the form of Ansem, Seeker of Darkness in order to defeat Roxas. Seeing the evidence of Riku's devotion to Sora, DiZ slowly begins to lose his lust for revenge, hiding it behind laughter. Still, DiZ continues with the plan, placing Roxas into a digital facsimile of Twilight Town where he can live out his days under observation, until it is time for him to reunite with Sora.
Kingdom Hearts IIEdit
A week later, DiZ, Riku, and Naminé still co-operate with each other to restore the memories of Sora, Goofy, and Donald. However, Naminé begins reaching out towards Roxas, to DiZ's annoyance, while Riku only intends to stick around until Sora awakens. Nonetheless, Roxas fuses with Sora eventually, though Riku does not dispose of Naminé as DiZ had wanted. The three go on their separate paths, while Sora goes off on his journey. Unfortunately, Sora does not function the way DiZ wanted, and all of his plans fall apart. It becomes increasingly obvious to the former ruler that the heart is something he cannot truly understand.
At this point in the story, DiZ has clearly established prejudices against Nobodies. DiZ, unlike Riku, has no pity for Roxas when they abduct him (DiZ simply saying "It's the fate of a Nobody"). Later, when Roxas demands answers to his questions, stating that he has a right to know, DiZ coldly says, "A Nobody doesn't have a right to know, nor does it even have a right to be", and tells Riku that he doesn't care what happens to Roxas as long as Naminé succeeds in restoring Sora, Donald, and Goofy. Similarly, he does not care what becomes of Naminé, and only keeps her around due to her usefulness, telling Riku that he can dispose of her once Sora's memories are completely restored. This prejudice is likely due to his hatred of his former apprentices and the Nobodies that they have become. Apparently, he disregards the fact that Roxas and Naminé are special Nobodies who are truly capable of feeling human emotions. This prejudice is what leads Riku to dislike DiZ greatly, and serves as his reason for defying DiZ by deliberately letting Naminé go with Axel, and leaving DiZ once Sora has awoken. However, as the plot progresses, DiZ begins to see the consequences of his actions. DiZ laughs at Riku in his "Ansem" form at first.
DiZ eventually infiltrates Organization XIII's home world, The World That Never Was. He takes it upon himself to stop Xemnas by using his machine to digitally encode Kingdom Hearts, which Xemnas had been feeding human hearts in order to cause it to mature, so that he could extract its power and become a god. However, as he ascends the Organization's castle, he is attacked by Nobodies, but is saved by his old friend King Mickey. Mickey helps him to his feet, and, seeing no more reason to hide his identity, DiZ unmasks himself as Ansem. King Mickey asks Ansem why he didn't come to him before the situation got as bad as it did, but Ansem feels that the task of defeating Organization XIII was his because their leader Xemnas is the Nobody of his apprentice Xehanort. He also explains to King Mickey his part in causing Riku to take on his "Ansem" form, and the regrets he had concealed with his true identity. Ansem and King Mickey then set off to seal away Kingdom Hearts.
Finding a good spot, Ansem begins encoding Kingdom Hearts, but sees that the power of Kingdom Hearts is too great to be controlled. The machine begins to overload, and Ansem does his best to contain it, but his efforts become increasingly futile. Sora, Riku, Kairi, Donald, and Goofy, as well as Xemnas, arrive at Mickey and Ansem's location. Ansem throws bitter words and accusations at Xemnas, though Xemnas simply retorts that Ansem himself was the reason for the destruction wrought on the worlds. Ansem, admitting his errors, states that both he and his former apprentices know as little about the heart as they did ten years ago. As the machine then goes critical, Ansem gives King Mickey and Riku his final words, tells Sora that the rest is up to him, and sincerely apologizes to Roxas for all that he had put him through (though he does not know if Roxas could hear him). Though Sora and company want to save him, they have no choice but to let him do as his heart commands. The machine then explodes, apparently killing Ansem.
However, his sacrifice is not in vain, as the explosion completely purifies Riku of the presence of Ansem, Seeker of Darkness and undoes the majority of Organization XIII's work on Kingdom Hearts, while inadvertently summoning forth several hundred Heartless in the city below.
In reality, Ansem the Wise survived the explosion, but was transported to the Realm of Darkness with no apparent way to return. Shortly afterward, Ansem, dressed in his own black coat, is found by Aqua at the Dark Meridian. She and Ansem talk, and Ansem reveals that he is losing his memories. He begins explaining his past deeds, and mentions that a boy with a Keyblade has always saved the world from darkness. She asks him about the worlds, and he tells her how Sora will save the worlds from being engulfed by darkness, and recalls his experiences with the boy. He felt reassured as he hid his research data within Sora while he slept, so as to open the door and help those who are "crippled".
On Sora's TeamEdit
He may be gone but Ansem still assists the team when they are in deep trouble as his ghost.