Vexen

""If you remain bound by the chains of memory, and refuse to believe your heart... then you may as well throw your heart away. You're no Keyblade master, no master of anything - just a slave to twisted memories. Just like my Riku. You're not fit to exist.""

- ~Vexen

Vexen, the Chilly Academic (いてつく学究, Itetsuku Gakkyū?, lit. Freezing Scholar), is a quaternary antagonist in Kingdom Hearts: Chain of Memories. He is voiced by Derek Stephen Prince in English and Nachi Nozawa in Japanese.

Vexen is the fourth member of Organization XIII and wields a shield and ice powers. Originally planning on using Riku's power of darkness to eliminate the traitorous Marluxia, he later attempts to confront Sora with a replica of Riku before engaging Sora himself.

Kingdom Hearts Birth by Sleep
Before, Vexen was originally known as Even. He was one of Ansem the Wise's apprentices, along with Braig, Dilan, Ienzo, and Aeleus.

When Ventus arrives at Hollow Bastion, he is accosted by Dilan and Aeleus, but the Trinity Armor's arrival forces Ven to leave. As Dilan and Aeleus attempt to follow the young Keyblade wielder, Even arrives and muses on what would become of the castle. Aeleus realizes that Even is right, but as they return to the castle, Even notes that Ven's heart is devoid of darkness.

Sometime later, Even, who is looking for Ienzo, encounters Ven, who had rescued Ienzo from a swarm of Unversed. Grateful of Ienzo's rescue, Even explains Ienzo's story to Ven and directs the Keyblade wielder to the Outer Gardens to find Terra.

Between Birth by Sleep and Kingdom Hearts: 358/2 Days
Sometime after Aqua's defeat of Terra-Xehanort, Xehanort is inducted into Ansem's ranks and becomes Ansem's favorite pupil. However, Xehanort wanted to delve deeper into the mysteries of the heart and darkness and convinced Even and the others to aid him. Their research eventually caused them to banish Ansem to the Realm of Darkness. At some point, Even's heart was consumed by darkness, giving birth to Vexen.

Years later, after Marluxia's induction, Vexen confronted Zexion and demanded to know where Xemnas was. Zexion simply informed Vexen that Xemnas was at the Chamber of Repose, causing Even to storm away while pondering why Xemnas was always hiding.

Kingdom Hearts 358/2 Days
Vexen plays a relatively minor role in Kingdom Hearts 358/2 Days, but it should be noted that this role plays a major contribution to the game's central plot. Advisor and leader of the Castle Oblivion-based Replica Program that created Xion, he was present when the Sora replica joined Organization XIII. Vexen then was assigned on Day 12 to teach Roxas the art of reconnaissance in Twilight Town, and was later getting sent away to Castle Oblivion, where he used his Replica Program to create the Riku Replica in order to challenge Sora. The Chilly Academic was later exterminated by Axel at the Castle, for the purpose of raising Saïx through the ranks. All Vexen had left behind after his demise was knowledge, Axel later discovering a document he had composed about Naminé in the Castle That Never Was's library. This Nobody's only other mention in Kingdom Hearts 358/2 Days was when Xemnas revealed the truth about Xion before the surviving members of Organization XIII in Where Nothing Gathers.

Kingdom Hearts Chain of Memories
In Kingdom Hearts: Chain of Memories, Vexen appeared as an antagonist in both Sora's and Riku's storylines. He assists Lexaeus and Zexion in their efforts to stop Marluxia (whom Vexen loathed) and Larxene (whom he's not too fond of either) from overthrowing the Organization (Axel, the other Organization member residing in the castle, is deemed too unpredictable to be anything other than a liability). To do this, Vexen and his cohorts intend to use Riku's darkness to counteract Sora's light.

Vexen engages Riku in a cautious battle and uses the data he obtained from the fight to create a copy of Riku that lacks a heart. The replica is completely aware of his status as a fake, but unlike Riku, is not afraid to use the darkness. However, even with darkness on his side, the replica fails to defeat Riku.

Even with the failure, Vexen has other plans for the replica, and goes to Marluxia and Larxene. He offers the replica to test Sora, though first he would have to have his memories changed around. They then have Naminé manipulate the replica's memories until he thinks he is the real Riku. He also believes that he made a promise to Naminé like Sora had, and because of this conflict, the replica confronts Sora multiple times, saying that Naminé no longer needs Sora.

Involvement with Riku
Though Larxene and Axel also make brief appearances in the cutscenes of Reverse/Rebirth, Vexen is the only Organization XIII member in Chain of Memories who is fought as a boss in both Sora and Riku's stories. In Riku's story, Vexen confronts Riku on a mission to gain data on Riku without him noticing. When the battle is over, Vexen reveals his true objective and reminds Riku of the Darkness inside him. Vexen flees, taunting Riku. It is later revealed that the "data" Vexen received from this battle is what created the Riku Replica. Vexen is never seen again in Riku's story, but it is mentioned by Axel and Lexaeus later on that he has indeed been slain.

Involvement with Sora
However, the replica soon spirals out of control, and Marluxia deems Vexen's experiment a failure. Vexen is insulted, saying that a neophyte like Marluxia has no right to speak to an elder like Vexen. However, Marluxia makes the threat of reporting Vexen's failure to "the Superior", and Vexen's fear of the Superior's power overrides his previous elitist attitude. Now with Vexen under his grip, Marluxia orders Vexen to eliminate Sora, much to his surprise.

Vexen then confronts Sora, saying that Sora is in debt to him for reuniting him with Riku. Sora is angered, and the two battle. However, as they fight, Vexen delves deep into Sora's heart, and creates a world card forged from the memories in the other side of his heart. Vexen tells Sora that if he wants to truly defeat him, then he must enter that world.

Meanwhile, Marluxia, Larxene, and Axel note that Vexen is jeopardizing their plans, for if Sora discovers the other side of his heart, then he will no longer be of any use to them. They then decide to have Axel eliminate Vexen before permanent damage can be done.

Sora discovers that the new world is Twilight Town, which is strange because Sora has only visited places in his memories, and he has no memories of Twilight Town. Even so, Sora feels a sense of familiarity from this place. He explores the town until he arrives at the gates of a mansion. Vexen makes his appearance there, asking Sora if the familiarity of this place or his memories of Naminé is true. Sora doesn't understand what he means, and Vexen sees that he is but a slave of his memories, just like "his Riku". Sora is again angered, and battles Vexen one more time.

Vexen loses again, and seeing that Sora is too dangerous, plans to tell him of Marluxia's plot and, by extension, Roxas's existence. Before he can fully explain, however, he is struck by Axel, who embeds a chakram into Vexen's body. Vexen begs to be spared, but Axel tells him he can be "nothing instead of just being nobody" and snaps his fingers, detonating the chakram and incinerating Vexen, making him the first member of Organization XIII to die.

Kingdom Hearts II
Vexen's spirit returns as an Absent Silhouette in Agrabah after Sora examines the Peddler's revived shop. If Sora investigates the Silhouette, Vexen challenges the Keyblade wielder for a rematch, but is eventually slain. Sora can also challenge a data replica of Vexen in the Garden of Assemblage.

Kingdom Heart Dream Drop Distance
Sometime after Vexen and the other members of Organization XIII were defeated, Vexen was revived as is original self, Even. Other members that were revived included Dilan, Ienzo, Aeleus, and Lea, but Braig and Isa were nowhere to be found. After the revived members regained their consciousness, both Even and Ienzo investigated on where Braig and Isa were. They believed that if their Nobodies were defeated, their Somebodies should've return to where their Hearts were extracted. However, the two believed if they weren't revived, Braig and Isa could've sent to Traverse Town.

Trivia

 * In Kingdom Hearts: Chain of Memories, Vexen was stabbed from behind by Axel and then struck down with another blow from his chakrams. In Kingdom Hearts Re:Chain of Memories, Axel throws a flaming chakram, snaps his fingers and a large burst of fire sets him ablaze. Lexaeus' and Zexion's death scenes were altered as well.
 * In Where Nothing Gathers, the room hosting the Organization's thrones, Vexen has the lowest of all thirteen seats.
 * Vexen shares Japanese voice actors with Professor Hojo of Final Fantasy VII, another scientist character who worked on creating people.
 * Vexen is one of five Organization members who are shown suffering the wounds that kills them rather than just fading into darkness outright. In this case he was impaled through the torso with a chakram and then immolated by Axel.
 * Vexen's Limit Break from Kingdom Hearts 358/2 Days, as well as his Sleight in Kingdom Hearts: Chain of Memories, is named after the attack Shiva (a summon from the Final Fantasy series) performs. The three main characters from Kingdom Hearts Birth by Sleep also have a Mode Change named "Diamond Dust".
 * Three of Vexen's weapons are named after glacial periods.
 * In Kingdom Hearts: Chain of Memories, Larxene mockingly called Vexen "Vexie." This, however, was changed in Kingdom Hearts Re:Chain of Memories. A similar case occurred with Zexion.

Some info thanks to Kingdom Hearts Wiki