Ansem the Wise
Ansem the Wise, referred to as Master Ansem, Lord, or His Lordship by his apprentices, is the sage-king of Radiant Garden during Kingdom Hearts Birth by Sleep. In order to protect his people from the darkness, he spent much of his time studying the heart with his apprentices, Braig, Dilan, Even, Aeleus, Ienzo, and Xehanort, but was eventually exiled to the Realm of Darkness when they were corrupted by that same darkness. He himself gained dark powers, renaming himself DiZ, short for "Darkness in Zero", and setting out on a quest to avenge himself against his apprentices.
Before Kingdom Hearts Birth by Sleep
From a time before the Unversed began invading the Realm of Light, Ansem was the king of Radiant Garden, the "city of light". He governed the Radiant Garden in peace and studied ways to make it safer with his five apprentices, Braig, Dilan, Even, Aeleus, and the orphan Ienzo.
Kingdom Hearts Birth by Sleep
In the aftermath of the battle between Eraqus's pupils and 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 lying next to him, and worked to help him regain his lost memories. Later, on a walk with Ienzo, Ansem spots Xehanort and Braig deep in conversation.
Between Kingdom Hearts Birth by Sleep and Kingdom Hearts Chain of Memories
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 simple psychological tests on Xehanort as a start. 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. Even so, Ansem eventually constructed a large laboratory in the basement of his castle, encouraged by Ienzo.  One of Ansem's experiment subjects was known as Subject X, an imprisoned amnesiac girl who could only recall three facts of her life. Unknown to him, Braig secretly released Subject X,  making Ansem order all experiments cease, as he feared that they had caused her disappearance  due to the experimentation snowballing out of control.  Unknown to Ansem, Xehanort believed that he had hidden Subject X from him. 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, King Mickey Mouse, soon arrived. The two rulers became great friends, conversing over many subjects, and their companionship began to heal Ansem's heart. Around this time, Xehanort had continued the experiments in secret, accidentally creating Heartless and presented his work in the hopes of praise. However, Ansem forbade an further experimentation with the Heartless as they threatened the stability of the worlds.  Ansem even accepted the boys Isa and Lea on as new apprentices.  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.
Ansem was eventually usurped by Xehanort, who lead Braig, Dilan, Even and Aeleus in banishing him to the Dark World, so they could continue their research without his interference. 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 newfound 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. He also learned of the Organization, whose membership consisted of the Nobodies of his former apprentices as its core members.
Kingdom Hearts Chain of Memories
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. All the while, he disguises himself as Ansem, Seeker of Darkness.
Once Riku is reunited with King Mickey, DiZ uses the simulation of Twilight Town to briefly separate them. Riku then reveals that he's learned to tell the odor of darkness emanating from its users apart; DiZ does not smell like the real Heartless, which has a foul odor of darkness. His ruse revealed, DiZ introduces himself and instructs Riku to meet Naminé to help decide his fate. Meanwhile, DiZ reunites with Mickey, but keeps his identity concealed, wanting his old friend to dissuade him from revenge.
After Riku decides not to lock his darkness away, DiZ provides him and Mickey with the black coats to keep them undetected from Organization XIII, 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.
When Riku and Mickey set off from Castle Oblivion, DiZ appears to them at the crossroads in the grassy plain, asking which path they will take: the road to light or the road to darkness. 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 Days
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 his shame behind laughter. Still, DiZ continues with the plan, placing Roxas into a digital copy of Twilight Town where he can live out his days under observation, until it is time for him to reunite with Sora.
Kingdom Hearts II
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. Sometime before Sora awakens, DiZ hides his research data on the hearts he found within Sora inside of Sora's heart as "Ansem's Code".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.
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 for himself. However, as Ansem ascends the Organization's castle, he is attacked by Nobodies and Heartless, but is then 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 and Ansem himself vanishes completely.
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 involuntarily summoning forth several hundred Heartless in the city below.
Having been transported to the Realm of Darkness following the explosion, Ansem, dressed in his own black coat, is found shortly afterwards by Aqua at the Dark Margin. Having lost his dark powers and much of his memory of the first time he was imprisoned, Ansem is just as a trapped as the lost Keyblade Master. Reflecting back on his actions and his former grudge, Ansem 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 twice saved the worlds from being engulfed by darkness, and recalls his experiences with the boy. He also adds to Aqua that he has hidden his research data within Sora while the boy was regaining his memories, and hopes that Sora will one day use this to open the door and save him and those connected to his heart.
Kingdom Hearts Re:coded
Ansem is mentioned by Mickey as "a friend who said that the heart is so much more than any system."
Kingdom Hearts 3D: Dream Drop Distance
After Riku frees Sora's heart from Ventus's possessed Keyblade Armor, he ends up on a projection of Destiny Islands within his heart. There, after meeting projections of Roxas, Ventus, and Xion and finding a bottle in the surf, Riku encounters a data projection of Ansem. He tells Riku about Ansem's Code, research data contained in the bottle that he had hid inside Sora while he was asleep. He muses on Sora's ability to connect with others' hearts, and how it may be able to bring back those that have been lost or no longer exist.
Kingdom Hearts III
Ansem and Aqua's peace is interrupted by Xehanort's Heartless, who demands the location of Subject X. Ansem denies knowing her whereabouts, resulting in the Keyblade-less Aqua coming to his defense when Xehanort attempts to kidnap him. Not wanting any further harm to come to her, he leaves with Xehanort willingly while Aqua is imbued with darkness by Xehanort and tossed into the ocean.
Ansem is brought to the Old Mansion by Xehanort's Heartless, so he can reveal the data on his computer. He begs Xehanort's Heartless to cease, as he does not wish for more victims. Xehanort's Heartless reminds him of the children sacrificed in their research; even his mistreatment of Roxas and Naminé. Ansem states he wishes to make amends for not respecting their right to exist. Suddenly, Pence, Hayner and Olette help Ansem escape; Xehanort's Heartless is blocked from following by Dusks.
A safe distance away, Ansem recognizes the trio as Roxas' friends. They are surprised by Vexen, whom Ansem realizes sent the Dusks. Vexen reveals he wishes to atone for his past crimes as well, sabotaging the Organization from within.
Vexen convinces Demyx to assist them; Ansem is safely chauffeured back into his old lab. Ansem greets Ienzo, who makes amends with him. Demyx interrupts the reunion to remind Ansem of the replica Vexen gave them. Ansem and Ienzo transfer Roxas's data into the replica.
Through Demyx, Riku later provides Ansem with the now-blank replica that once held Dark Riku's heart. Together with Ienzo, and completed Even, Ansem is able to add his data on Naminé to it, allowing her lost heart to reside in it. With both her and Roxas revived, Ansem succeeds in atoning for his past misdeeds towards them.
Ansem the Wise originally wore a periwinkle labcoat, identical to those worn by some of his apprentices (Even, Ienzo, and Xehanort), save for the color. Underneath this, he wore a dark grey vest, white collared shirt, and a violet ascot, as well as black pants and shoes. He also draped a red sweater or coat of some sort over his shoulders. Ansem's hair was blond and worn long and slicked back. He also sported a short mustache and goatee. His eyes are bright orange.
As DiZ, Ansem dons a much more complex outfit. His face is covered in red bandages, save for his right eye and mouth, and he tied two intersecting belts around his head, one passing over where his left eye should be. He wears a long, red cape with a quadrate cross cut taken out at the chest, the edges of which are yellow. This cut exposes a similarly oblong white symbol on a pitch-black shirt. Over-top this shirt, DiZ wears what seems to be grey, segmented armor over his sides and abdomen. His legs are covered by a long, black cloth lined with yellow, which itself has several brown straps pinned onto the front, seemingly for no purpose other than to create yet another angular pattern. On a yellow sash that hangs around DiZ's waist are three brown pouches, descending downward diagonally on his left hip. These pouches are all identical, all sporting the same design on the top flap, and all closed by a darker brown strap that attaches to a silver button on the right ride, a grey tassel hanging from each button. Like Ansem and his coat, DiZ wears a dark colored scarf around his neck. He also has light colored sleeves.
Notably, Ansem's skin is darker when in his "DiZ" disguise, and his beard is not present, despite his chin being mostly visible. When he removes the bandages, however, these changes disappear in a dark vapor, suggesting that Ansem used his dark magic to artificially change his appearance. His eyes appear to be a much darker shade of amber orange before the DiZ persona, but this is likely an optical illusion caused by the difference in skin color of Ansem and DiZ. In fact, his skin and eye color while disguised as DiZ are very similar to those of "Ansem, Seeker of Darkness". Interestingly, Ansem retained the amber eyes after discarding his DiZ disguise, and his ears were shown to have developed slight pointy, similar to Master Xehanort.
Upon his first appearance in Kingdom Hearts Chain of Memories, Ansem is disguised as "Ansem, Seeker of Darkness", and thus shares many personality traits with the Heartless while harboring his hidden agenda of destroying Xehanort and Organization XIII. Once he reveals himself as DiZ, he becomes much less sinister, and appears to be genuinely interested in helping Riku and Sora.
As himself, Ansem the Wise, true to his name, is a wise sage-king, even though he has been overthrown. It is revealed that his betrayal by Xehanort was what turned him from a selfless man to a lonely, bitter and rather self-serving individual bent on revenge on Xehanort and apparently caring nothing for the fate of the universe. One notable personality trait that he exhibits in Kingdom Hearts II is his clear dislike of Nobodies in general, likely due to his rage at his former apprentices and the Nobodies which they have become. His prejudice is so deep that he orders Riku to eliminate Naminé once they are finished with her, despite everything she had done for their noble cause (although Riku refuses). His attitude, however, begins to change when he witnesses Riku's transformation into "Ansem" in order to wake Sora up: he first laughs at Riku and mocks him for using the name "Ansem", but later confides to Mickey that his heart was aching greatly at the mere sight of Riku and he began to lose his desire for revenge.
By the end of Kingdom Hearts II and Kingdom Hearts: Birth by Sleep, Ansem seems to have reverted to his selfless ways and willingly sacrifices himself to stop Xemnas from succeeding with his artificial Kingdom Hearts. He apologizes to Roxas via Sora, and expresses to Aqua his remorse for everything he put Sora, Roxas, and Riku through while they are in the Dark Margin, openly hoping that Sora will ultimately open the Door to Light and right all of his own wrongdoings during his quest for revenge. He ultimately feels remorse for not respecting Roxas and Naminé's right to exist, as they became different people from Sora and Kairi due to lacking their memories.
In Kingdom Hearts III, Ansem remains deeply remorseful over the sacrifices that were made for his research, such as the numerous children who became his victims, and remains determined to atone for his mistakes. Ansem shows he was unwilling to let others suffer in his defense as he complied with Xehanort's Heartless to come quietly if he spared Aqua. Ansem was further able to put his past grievances with his apprentices behind and make peace with them, rekindling the bond they once shared.
Ansem the Wise has no combat abilities to speak of, but since gaining the power of darkness during his banishment, he has been shown to be able to do at least three things. In one of his reports, Ansem stated that after he discovered his new-found powers, he gained the ability to use Corridors of Darkness for travel, though he admits that it was difficult to control initially. This was shown after Riku defeats Roxas; Ansem arrives shortly to take Roxas with him. To avoid being recognized, Ansem has also displayed the ability to change his outward appearance to conceal his identity. In Kingdom Hearts Chain of Memories and Kingdom Hearts Re:Chain of Memories, he uses this ability to become Xehanort's Heartless to guide Riku.
Ansem also shows the ability to use darkness to repel would-be attackers; while testing Riku in his disguise as Xehanort's Heartless, he merely had to raise a hand to create a small force to push back Riku. He also demonstrated the ability to craft World Cards from Riku's memory, although altered to only show the Villains and Heartless i.e. "dark beings". After his Encoder explosion sent him to the Dark Margin, Ansem lost his dark powers, or at least the memory of how to use them.
Ansem is also a technical genius, having created a functioning digital replica of Twilight Town, as well as a facility there. His facility in Twilight Town is a very high-tech laboratory, filled with computer screens and several suspended-animation pods. He also created the Kingdom Hearts Encoder, designed to undo Kingdom Hearts.