Vexen, the Chilly Academic (いてつく学究 Itetsuku Gakkyū ), is the Nobody of Even and the Rank IV member of the original Organization XIII. He controls ice and uses it to command storms and blizzards, freezing his opponents solid, while defending himself with his massive shield.
Vexen was the highest ranking member of the Organization inside Castle Oblivion, but ironically he is the first Organization member to be eliminated. He created the Riku Replica and uses the Replica to carry out several experiments on Riku and Sora. Vexen appears in Kingdom Hearts: Chain of Memories and he appears in Kingdom Hearts 358/2 Days as a playable character in Mission Mode.
 Between Kingdom Hearts Birth by Sleep and Kingdom Hearts 358/2 Days
Even, along with the other apprentices of Ansem the Wise, loses his heart in the course of their experiments into the nature of the heart and its darkness. Even's Nobody, Vexen, is born in the aftermath, and he and the Nobodies of the other apprentices become the founding members of Organization XIII.
Vexen serves as the Organization's leading researcher, heading their investigations into memories and creating the Replica Program.
 Kingdom Hearts 358/2 Days
After Roxas is inducted to the Organization, Vexen uses his top-quality Replica, designated "No. i", to copy Roxas's abilities. Xion is introduced to the Organization as Number XIV, and Vexen determines that she should be kept apart from Roxas for ten days, until she proves to be functioning properly.
On Day 12, Vexen takes Roxas on a mission to Twilight Town to introduce him to reconnaissance duties. Although he addresses Roxas condescendingly, he informs Roxas of the existence of many worlds and explains the importance of exploring worlds for the Organization.
 Kingdom Hearts: Chain of Memories
When Marluxia and Larxene begin manipulating Sora through Naminé's ability, Vexen, Lexaeus, and Zexion decide to try setting Riku, who has appeared in the castle's basements, to counter Marluxia's plot. Vexen engages Riku in battle, hinting towards the nature of Nobodies and the existence of Xemnas and taunting Riku about his nature as a being between darkness and light. Although he loses, Vexen uses the data he obtains from the fight together with the second Replica. He then has the Riku Replica challeng Riku in battle, during which he uses Riku's full power of darkness, but he is defeated.
Even with the failure, Vexen has other plans for the replica, and goes to Marluxia and Larxene. He offers to use the replica to test Sora, although they insist on changing his memories. They have Naminé manipulate the replica into thinking he is the real Riku and that he made a promise to Naminé, something that Sora also remembers doing. The Riku Replica confronts Sora multiple times, but he is defeated each time.
After the replica loses twice to Sora and fails to confront him a third time, Marluxia deems Vexen's experiment a failure. Vexen is insulted, but when Marluxia makes the threat of reporting Vexen's failure to "the Superior", Vexen reluctantly agrees to face Sora himself. He meets Sora at the entrance to the 11th floor and states 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 Roxas's memories, in the other side of Sora's 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 Roxas's existence, then he will no longer be of any use to them. Marluxia sends Axel to take care of Vexen.
After exploring the memory-Twilight Town, Sora encounters Vexen at the gates to the Old Mansion, where Vexen asks Sora whether the strange familiarity his feels for the town or his memories of Naminé are true. Sora doesn't understand what he means, and Vexen remarks that he is enslaved to his memories, just like "his Riku". Sora is again angered, and battles Vexen once again.
Vexen is again defeated, and seeing that Sora is too dangerous, tries to tell him of Marluxia's plot and, by extension, Roxas's existence. Before he can fully explain, however, he is interrupted by a strike from Axel. Vexen begs to be spared, but Axel tells him he can be "nothing instead of just being nobody" and snaps his fingers, incinerating Vexen.
 Other appearances
 Kingdom Hearts: Chain of Memories manga
In the manga, Vexen creates a set of Vexen Replicas (failed replicas, according to the replicas themselves) and has a grave which the Riku Replica visits before leaving Castle Oblivion.
Vexen wears the basic Organization uniform of a black coat with like-colored boots and gloves, though the sleeves of Vexen's cloak are a bit longer than normal and partially cover his hands. His cloak is also the longest out of all the members having it almost like a gown that covers his boots. Vexen sports platinum blond hair (which was a somewhat brighter shade of yellow in the Gameboy Advance version of Chain of Memories), worn long with two slightly shorter (but still long) bangs that frame his face. He also has rather large, bright green eyes.
Vexen approaches problems like an intellectual, with a hypothesis and a method to test those hypotheses. He appears to view life as a grand experiment, suiting his title as "The Chilly Academic". He appears very much like the stereotypical 'mad scientist.' More than any other member, save Xemnas, he considers his numerical rank a literal one and will often deride members with lower numerical rankings than his; he reminds Marluxia of his superior rank when Marluxia ordered him not to "disappoint us" again and addresses Zexion by his inferior rank when Zexion lamented Vexen's impatience, but is often chastised by others for this behavior.
Vexen also displays a fear of Xemnas, immediately bowing to Marluxia's demand to destroy Sora when Marluxia threatened to tell Xemnas of the failure of the Riku replica. He also feared to approach Xemnas while the latter was in the Chamber of Repose, and took Zexion's suggestion to approach Xemnas there as a joke.
Ironically, Vexen's throne is the lowest of the Organization in their meeting chamber known as Where Nothing Gathers. This is mostly due to his recurring incompetence and preference of not sullying his hands like Zexion, though the latter can easily hold his own in battle. This also reinforces the trait of cowardice in Vexen, displayed through his fear of Xemnas and his fear of death.
In the Kingdom Hearts: Chain of Memories manga, Vexen appears as a character who thinks that he doesn't get enough respect from the aboveground members. He is also met by Sora, Donald, and Goofy right after they confront the Riku Replica for the first time, while in the original game, they fought the Riku Replica twice before fighting Vexen two floors later.
Vexen is one of the most prolific Organization members in battles, appearing as an opponent three times during Kingdom Hearts: Chain of Memories. He fights with a large blue shield, which he uses to fend off frontal attacks, making it necessary to attack him from behind. He is also able to use the shield offensively, akin to Goofy, but further enhances the style with the spikes lining the edge of his shield. He can use his cryokinetic abilities to blast his enemy with ice from his hand, and can call forth large chunks of ice from the ground to freeze his opponent solid. He also has the power to create slippery ice patches on the ground, or summon icicles that rise from the ground and follow their target for several seconds. Finally, he can create an unavoidable blizzard that can be resisted only with a card break. In the remake, Vexen can conjure up a sword out of ice and whack his opponent several times with it before performing a final shockwave which destroys the sword and ends the sleight.
Like several other members of Organization XIII in Kingdom Hearts: Chain of Memories, Vexen moves by levitating, sometimes several feet into the air. Also, in the English version of Kingdom Hearts: Chain of Memories, Vexen has a high-pitched laugh similar to that of Kefka Palazzo from Final Fantasy VI while in battle. This is not heard in Kingdom Hearts Re:Chain of Memories.
In Kingdom Hearts II Final Mix, Vexen, as well as other members of the Organization that have already been eliminated, can be fought. Vexen's shield makes him completely invincible in this game, but the shield has an HP bar of its own and can be temporarily destroyed, allowing Sora to attack Vexen directly. Vexen has the additional power in this battle to create a copy of Sora's Anti form to aid him in battle. Vexen still has his "freeze" attack when a blizzard freezes Sora, immobilizing Sora while Vexen gains data, but it can be stopped by a Reaction Command. He also has the icicle attack from before, but also returns with a new attack when he makes two spinning blades of ice and throws them at Sora. In the data battle against Vexen, he has a new attack when he throws giant chunks of ice at Sora.
Vexen also has the ability to collect and manipulate Data. This ability allows Vexen to create Replicas, such as the Riku Replica, and his numerous Vexen Replicas. Not much else is known about this ability. It can be assumed that he uses his shield, Frozen Pride, to absorb attacks, and collect data simultaneously.
In Kingdom Hearts 358/2 Days, Vexen has a Limit Break called Diamond Dust. During Diamond Dust, Vexen summons three large mounds of ice to come up from the ground to strike enemies, dealing ice damage to and freezing all struck enemies. Enemies that are frozen take three times as much damage while frozen. During Vexen's Final Limit, Vexen's three mounds of ice are replaced by one larger glacier. This deals greater ice damage to enemies, and temporarily makes them gradually lose HP. It also freezes them for awhile.
Initially appearing as a powerful foe to Sora, Vexen is revealed to be one of the weaker members of Organization XIII alongside Demyx. This is indicated by the fact that no one in the Organization seems to show Vexen any respect despite his high rank. Marluxia and Larxene seem to frequently poke fun at him with jokes and sneers, with Larxene going as far as to call him a "half-baked good for nothing". Vexen is also easily eliminated by Axel, who fought evenly with Marluxia. In Riku's story, Vexen seems to be below Zexion and Lexaeus in terms of authority as he is the one doing all of the work while the other two hide in the basement and direct him.
Vexen, like Goofy, carries a shield into battle. His shield Frozen Pride is pointed, and is colored blue and silver. He can use it to block attacks, but it is also used to slash foes. If turned upside down, the shield has the vague shape of the Nobody symbol. In Kingdom Hearts Re:Chain of Memories it can be changed during the sleight "Slide Break," where he freezes ice onto the shield to form a gigantic ice sword that he wildly slashes with before smashing the blade onto the foe. However, his Absent Silhouette does not use it to attack, instead relying on his data replicas and ice magic for his offense, and uses his shield to block any attacks during battle. However, the Silhouette's shield has its own small health bar, and breaks after a while, and leaves him vulnerable until another shield is summoned. In Kingdom Hearts II Final Mix, Goofy can obtain a similar shield through synthesis.