Zexion

Zexion, the Cloaked Schemer, is number VI in Organization XIII, and the youngest of the original six members. He appeared in Kingdom Hearts: Chain of Memories as a participant in the struggle within the Organization between the newer and veteran members. He is also central to the plot to use Riku as a puppet in Castle Oblivion.

Unlike the other Organization members, Zexion's weapon was originally unknown, and his talents lie outside of normal combat. In an effort to keep Zexion's weapon unknown, his gravestone in the The World That Never Was was destroyed by the makers of the game. This was because all the gravestones, besides the title of the member, also depicted their weapon of choice. It was later revealed that Zexion's weapon was a large book.

Discovering Riku
After Sora had been lured into Castle Oblivion, Zexion discovers two other scents deep in the dungeons of the castle. One of them belongs to a Maleficent formed from someone's memory, having been given life by Castle Oblivion's unique properties. Originally, he mistakes the other scent to be that of "The Superior", an (at the time) unnamed entity within the Organization. Eventually, however, Zexion and his cohorts determine that the scent belongs to Riku, who had appeared in the Castle by force of will after Sora arrived. Vexen hypothesizes that the darkness in Riku's heart caused Zexion to mistake him for "the Superior" (Xemnas), calling the effect "fascinating". Specifically, Riku's heart is plagued by the presence of Ansem. The relationship they share is that "Ansem" and Xemnas both originated from Xehanort; "Ansem" is Xehanort's Heartless, and Xemnas is his Nobody.

Opposition to Marluxia
Zexion, as well as Lexaeus and Vexen, come to see that Marluxia, keeper of Castle Oblivion, intends to take control of Organization XIII, with the help of Larxene, by using Sora as their puppet. Though they see that they can use Riku to counter Sora, they decide to oppose Marluxia's plot by having Vexen investigate Marluxia under the guise of offering to help using the Riku Replica he creates with combat data from the real Riku. However, Vexen is eventually killed on Marluxia's orders by Axel, whom Zexion and Lexaeus had been unsure about trusting. Axel later tells them he had performed the deed only to gain Marluxia's trust, and continues to cooperate with Zexion in tracking Sora and Riku.

Seeing that Vexen's efforts to derail Marluxia's plan had failed, Zexion sends Lexaeus to sway Riku into joining them, but Riku refuses to allow darkness to control him and kills Lexaeus in combat. With the last of his strength, Lexaeus tries to envelop Riku in darkness, but Riku breaks free and continues ascending Castle Oblivion.

Confronting Riku
After Lexaeus dies in his failed attempt to subvert Riku's resistance to darkness, Zexion discusses the Organization's fate with Axel and learns that Larxene has been killed as well, bringing the number of Organization members eliminated to three. Axel tells him that he himself was defeated by Sora as well, and that Marluxia is almost certainly the next to go. Zexion decides that with the possibility of Marluxia dying soon, Riku is no longer needed, and heads off to intercept him.

Zexion appears before Riku while he is between floors of the castle, shortly after the death of Marluxia. As the force of Marluxia's death shakes the castle, Riku halts with surprise and comments how a very strong scent in the air has died. Zexion materializes in front of him and tells him that Sora has just defeated Marluxia. Riku is surprised to learn that Sora is also in the castle, but Zexion tells him he would never be able to face him after having such darkness in his heart. He gives Riku a World Card to Destiny Islands, and Riku winds up facing a Darkside Heartless. After he defeats it, Zexion reappears, and continues to play on his guilt, telling him Sora will always oppose him as a host of the dark. Sora then appears himself, brandishing his keyblade and accusing Riku of being corrupt. He envelops Riku in light, rendering him unable to do anything. Instead of drowning Riku into helplessness, however, the predicament causes him to draw on the power of darkness within him to free himself of the oppressing light and strike out at Sora. He wounds Sora, whose disguise falters to reveal Zexion, who had been using his powers of illusion to deceive Riku. Zexion, astounded at Riku's ability to wield the darkness so well, vanishes.

After escaping from Riku, Zexion appears in a room, where Axel and a companion are waiting. In a panic, Zexion thinks Axel's companion is Riku himself, but then realizes it's only the replica. Axel and the replica then approach Zexion, and Axel directs the replica's attention to his lack of wholeness. He tells him that if he can find strength that the real Riku doesn't have, he can become a whole person of his own, rather than a mere puppet or whole copy of Riku. Axel then suggests that he should start by eliminating Zexion. Zexion is horrified by Axel's suggestion, and Axel says that it would be far more entertaining to watch Riku and Sora than to aid Zexion any longer. The replica grabs Zexion by the throat and drains him of his lifeforce, leaving Axel as the only survivor of Castle Oblivion among the Organization.

Characteristics
Zexion is distinguished from the other members of Organization XIII by his approach to dealing with his enemies; namely, he does not fight them outright, but uses schemes and illusions to bring them down. He wields the element of "illusion" and has no visible weapon during the game, although he does have a weapon that is revealed in Kingdom Hearts II: Final Mix. His weapon panel in the Proof of Existence (an area of the Castle that Never Was displaying the Organization members' weapons and titles) is shattered in the original, but shows through cutscenes in KH:RCOM that his weapon is, in fact, a book which attacks through a flurry of pages and trapping people within it. Zexion also has the ability to identify the "scents" of others, with great accuracy and at great distances. The "scent" he smells may be based off other things than mere odor, and is affected by presence of darkness in the heart, an entity's power in the castle, and other factors.

Zexion stands out in Kingdom Hearts: Chain of Memories as the mastermind behind the actions of those members who are actual loyal to the Organization. He is aware to the Organization's decline, calling it "deplorable", and is especially harsh with those members who he feels to be arrogant.

Origins
Zexion is the Nobody of Ienzo, who was the youngest of Ansem the Wise's original six apprentices. Ienzo was the apprentice who tried to persuade Ansem to go further with his own research, and helped Xehanort complete the research into the secrets of the heart once Ansem refused to continue experimenting. He, along with Xehanort and Even (the original name of Vexen), is one of only three apprentices to be mentioned by name outside of the listing in the Secret Ansem Reports.

Trivia

 * Zexion is notable for being the only Organization member that originally couldn't be battled in Kingdom Hearts: Chain of Memories. However, in both Re:Chain of Memories, he can be battles in Riku's storyline. He also appears in Kingdom Hearts II: Final Mix as an optional boss character.
 * In the japanese version, Zexion was voiced by popular seiyuu Akira Ishida, whose last involvement with SquareEnix was to voice Makenshi of Final Fantasy Unlimited.
 * Zexion in the manga is usually seen cooking and is killed by the real Riku.

Videos
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Zexion (Re:CoM battle)

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Zexion (Final Mix+ battle)