Between Kingdom Hearts Birth by Sleep and Kingdom Hearts
Terra-Xehanort assumes the name of his master, Ansem the Wise, and he and his fellow apprentices banish Ansem to the Realm of Darkness. Their subsequent experiments cause them to lose their hearts in darkness, and Xehanort's being is split into a Heartless and a Nobody, the Heartless being Ansem, Seeker of Darkness, while the Nobody is Xemnas. The newly-born Xemnas then goes on to recruit the Nobodies of Ansem's other apprentices, and together, they form Organization XIII. While Xemnas's goal is ostensibly to summon Kingdom Hearts, he also seeks to gather thirteen vessels, each to bear a piece of Master Xehanort's heart.
The Organization's final member piques Xemnas's interest greatly, as he is the Nobody of a Keyblade wielder. Xemnas tracks down the young boy in Twilight Town and names him Roxas, taking him to his new home in The World That Never Was where he introduces Roxas to the rest of the Organization.
Xemnas arrives at Hollow Bastion shortly after recruiting Roxas, in order to observe Sora, Roxas's original being. The Princesses of Heart sense his incredible power, and Sora, Donald, Goofy, and the Beast go to the Chapel to investigate. Xemnas walks through Sora, intangible, causing Sora to experience many of his memories in rapid succession. Sora asks who the man is, and Goofy wonders if it is Ansem, and though Xemnas says he is familiar with the name, he does not clarify how or who he is. He then launches an energy attack at Sora, who manages to barely deflect it. Xemnas remarks that Sora looks like Roxas, and is not complete, but this only confuses Sora. He then attacks the Keyblade wielder, intending to test his power.
After an intense battle, he backs off, almost appearing to be defeated, but quickly abandons the facade. He explains that he finds Sora fascinating, and says that they will one day meet again and disappears, but not before making an ambiguous comment on how he is but "a mere shell".
Kingdom Hearts 358/2 Days
On Roxas's sixth day in the Organization, he meets Xemnas at the Dark Margin. Xemnas tells Roxas that he has been to see Sora and remarks that they look alike. Roxas asks Xemnas who he is, but Xemnas answers cryptically before asking Roxas if he knows his own real name. As he leaves the beach, Xemnas calls Roxas, "Sora".
Once Roxas is inducted into Organization XIII, Xemnas makes use of Roxas's ability to use the Keyblade to release captured hearts and use them to form Kingdom Hearts. At the same time, he uses his experiments on the Replica Program and creates a clone of Sora. The result is Xion, who Xemnas hopes to use as an alternative in case Sora proves useless to his plans. To that end, Xemnas plans to have Xion gradually develop Sora's abilities and then merge with Roxas to become a complete clone of Sora.
Xemnas sends the members of Organization XIII on their missions to further their plans while placing Xion in the care of Roxas. Xemnas eventually extends its operations to Castle Oblivion. With this fortress in his control, he sends five Organization members, Marluxia, Larxene, Vexen, Lexaeus, and Zexion, to conduct experiments on memory with the use of Naminé who is being held prisoner by the Organization. Xemnas places Marluxia in charge of Castle Oblivion and charges Vexen with the task of continuing the Replica Project. Xemnas was also the one who handed down the order to Marluxia to lure Sora to Castle Oblivion and experiment with the hero. However, once the Dusks brought back rumors of traitorous activities occurring in the castle, Xemnas sends Axel to neutralize the threat. Additionally, he instructs Axel to search for the Chamber of Waking within the castle to retrieve Ventus. This is due to Xemnas discovering that Ventus is a Keyblade wielder of old and the influence of Terra's memories.
Xemnas's plans falter when five Organization members are eliminated and Naminé escapes, but Xemnas continues his plans for Xion, placing Axel in charge of capturing her whenever she tries to escape the Organization. He would also repeatedly send Axel back to Castle Oblivion to continue searching for the Chamber of Waking, to no avail.
By the ninety-third day following Roxas's induction into the Organization, Xemnas gathers the rest of the Organization atop the Altar of Naught to see their Kingdom Hearts come into existence following the three months Roxas and Xion have been gathering hearts for it. Giving a speech about how they will regain hearts, but not be swayed by their recovered emotions, Xemnas ends their meeting and sends the other members back to their missions.
To fuel their Kingdom Hearts until it was "ripe" enough, Xemnas orders the other members to do all they can to create powerful Heartless for Roxas to defeat. When Xion disappeared, Xemnas gave an order not to go looking for her until the time comes. When Xion was brought back by Axel, Roxas questions him about her whereabouts and tells him that Xion will return to work the next day. Roxas suddenly has a flashback when Xemnas addressed him as Sora and wonders who Sora is, and all he said was that he's the connection. Later that day, Xemnas goes over plan with Xion and Roxas and reveals that although Xion has gained the ability to use the Keyblade as they intended, she has also gained an identity. As he was about to announce their plan a failure, he realized that it's because Xion has some of Sora's memories trapped which will prevent him from waking. Later, after Xion starts becoming more and more powerful—which drains Roxas of his own strength—he arranges for Roxas and Xion to fight, deeming it unnecessary to have two Keyblade wielders in the Organization.
On Day 354, after Xion escapes for the last time, Xemnas assembles the Organization in the Round Room, and he, Xigbar, and Saïx inform everyone of Xion's nature and the Replica Project. Xemnas sends Axel to retrieve Xion, and he brings her back to the Castle That Never Was before fainting. Xemnas reprograms Xion and sends her to absorb Roxas. However, Xion is defeated and absorbed by Roxas. As she fades away, Xion tells Roxas to free Kingdom Hearts and prevent Xemnas from getting it, as he would use it only to gain supreme power. Xemnas and Saïx meet in the Round Room to discuss Xion's defeat, and Xemnas orders the retrieval of Roxas.
Kingdom Hearts II
On the first day of Roxas's summer vacation, he has a dream of his earlier meeting with Xemnas at the Dark Margin. During the fourth day, Xemnas orders Axel to destroy Roxas if he doesn't return to the Organization, much to the latter's objections. He puts a halt to Xaldin's intimidation of Axel, suggesting a worse punishment for failure to comply: being turned to a Dusk. Unable to refuse, Axel relents and leaves to persuade Roxas.
True to Xemnas's earlier words, he and Sora meet again at Hollow Bastion, accompanied by Demyx, Xaldin, Xigbar, Luxord, and Saïx. Sora seems not to recognize Xemnas, and the Organization mocks the perceived weakness of Sora, only angering him. Afterward, they return to Where Nothing Gathers and discuss their plan to manipulate Sora. Thereafter, Xemnas remains at the Castle That Never Was while the other Organization members travel to various worlds to stir up the Heartless so that Sora can defeat them. After Saïx returns from Twilight Town, Xemnas thanks him for planting the seeds of doubt and anger in Sora's heart, which he believes will eventually bear fruit in turning Sora to their cause. When Saïx asks him about Axel, Xemnas merely mocks the traitor's desire for friendship.
During the early stages of the Battle of the 1000 Heartless, Xemnas comes to Ansem the Wise's computer room, where he opens a path to the Chamber of Repose. He enters the Chamber, located below the Heartless Manufactory, where he visits the armor of Aqua and briefly speaks with it. He then goes to the Great Maw, where he looks upon the Heartless army that had temporarily halted. King Mickey, upon seeing his face, finally remembers that Xemnas is the Nobody of Xehanort. Xemnas leaves the battlefield, but King Mickey gives chase and engages Xemnas in combat, while Sora, Donald, and Goofy are blocked from following by the Heartless.
After an exhausting battle, the three make their way to the cliff overlooking Villain's Vale, finally catching up with the King. Sora demands to know where Riku and Kairi are, but Xemnas claims to know nothing about Kairi while telling Sora to ask Mickey about Riku. Then, he vanishes into a corridor of darkness, followed by Mickey.
At some point, Xemnas shakes Mickey off his trail and returns to the World That Never Was. As the members discuss whether Sora can still be controlled, now that he knows the Organization is using him, Xemnas reassures them that beating Sora into submission will not endanger their plans, pleasing Xaldin.
Later, as the Organization members wonder what Axel was trying to achieve by sacrificing himself for Sora, Xemnas suggests that Axel may have touched Sora's heart and reawakened Roxas.
As Sora and company enter the Castle That Never Was, Saïx comes to Xemnas and asks if it is finally time to dispose of Sora, and Xemnas gives his consent.
When Ansem begins trying to seal Kingdom Hearts away with his Encoder, Xemnas comes to investigate and attempts to intimidate him by blaming Ansem's research for starting the twin plagues of Heartless and Nobodies. Regretful but defiant, Ansem finishes his work and shatters Kingdom Hearts, as his overloading Encoder explodes. In the chaos, Xemnas disappears.
Sora and the others corner him at the Altar of Naught, where Xemnas archly commands them to fetch him more hearts. He drags Sora into a battle with him in the phantom battleground of Memory's Contortion, echoing Riku's previous battle with Roxas.
Although Sora fends off Xemnas and is returned to the tower, the Superior is not yet defeated and retreats into the broken Kingdom Hearts in an attempt to exploit what little power it has left. Fortunately, the worlds create a door for Sora and the others to follow him into Xemnas's "The World of Nothing", a pocket dimension presided over by a massive, weaponized construct which has absorbed the remainder of The World That Never Was. His attacks manage to separate Kairi and Mickey from the others, but when Sora, Donald, Goofy, and Riku still manage to infiltrate his war-machine, Xemnas dons Master Xehanort's Keyblade Armor and attacks them head-on. Though he channels the powers of his fallen soldiers, he is still defeated and seemingly fades into darkness, and the heroes return to the Altar of Naught.
With the help of Naminé, Kairi, Donald, Goofy, and the King safely escape to Destiny Islands, but before Sora and Riku can follow, Xemnas attacks once more with his construct, having escaped The World of Nothing. Sora and Riku manage to destroy the construct and chase Xemnas back into his pocket dimension, where he launches a furious final assault, exploiting all his powers over light, darkness, and nothingness. After failing to slay them with a massive fusillade of Ethereal Blades, Xemnas is slain when Riku and Sora, together, use Sora's Keyblade to "unlock" Xemnas, finally destroying him.
Who I'd like to meet: