I'm a scientist. Experiments are what I do, yes.
Vexen, the Chilly Academic (いてつく学究 Itetsuku Gakkyū?, lit. Freezing Scholar), is the Nobody of Even, Rank IV within Organization XIII, and a reserve member of the real 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 was 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.
Despite his reconstruction as a complete being, Even opted to become a reserve member of the real Organization XIII as Vexen after being approached by Saïx to complete the Replica Program for their plans. Through his research, Vexen was able to finalize his work on the program, which yielded to the creation of replicas nigh-perfect enough to become full-fleshed humans. With his work complete, he sought to atone for his past misdeeds, first by saving his master Ansem the Wise from the clutches of Ansem, Seeker of Darkness in Twilight Town, and later tasking Demyx to deliver one of his replicas to Ienzo for the resurrection of Roxas. After the defeat of Master Xehanort, Vexen willing reverted back to Even.
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 reminded Marluxia of his superior rank when Marluxia orders 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.
Interestingly, 358/2 Days shows a more understanding side to Vexen's character; he is seen patiently guiding Roxas in the aspects of reconnaissance and gave the latter all the time he wanted to examine the town even more, before finally ending the mission with valuable advice. However, Vexen was irritated by Roxas only being able to "look, but not see"; in other words, making sense of the data he was gathering. He felt proud of himself when it looked like his teaching was paying off with Roxas - "small steps for small minds."
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.
In the Kingdom Hearts: Chain of Memories manga, Vexen appears as a character who thinks that he doesn't get enough respect from the upper floor members.
After reacquiring his humanity, Vexen, now having the heart to feel horror and shame for what he did, became determined to atone for his sins and only re-joined the Organization in order to help ensure its defeat. Although acting still as a passionate scientist, in truth Vexen was more than willing to risk his life to atone, which Demyx couldn't help but call him out for, and he uses his intelligence to successfully coax Demyx into joining him.
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 dirty blond hair (which was a somewhat brighter shade of yellow in the Gameboy 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.
A data recreation of Vexen can be fought during the New Organization XIII Event in the month of April.
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. Adviser and leader of the Castle Oblivion-based Replica Program that created Xion, he was present when the replica joined Organization XIII. Vexen then was assigned on Day 12 to teach Roxas the art of reconnaissance in Twilight Town, and was later sent away to Castle Oblivion, where he used his Replica Program to create the Riku Replica in order to challenge Sora.
Vexen was later exterminated by Axel to earn Marluxia's trust and for the purpose of raising Saïx through the ranks. All Vexen had left behind after his demise was knowledge that Axel later discovered in a document Vexen 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.
In Kingdom Hearts: Chain of Memories, Vexen has parts 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 was 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.
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.
However, they soon lose control of the replica, and Marluxia deems Vexen's experiment a failure. Vexen is insulted, saying that a novice like Marluxia has no right to speak to an elder like himself. However, when Marluxia makes the threat of reporting Vexen's failure to "the Superior", Vexen's fear of the Superior's power overrides his previous elitist attitude. Now with Vexen under his grip, Marluxia manages to persuade Vexen to eliminate Sora. In the remake, Vexen is instead put in his place by Marluxia by the latter reminding him of how Xemnas had put him in charge of Castle Oblivion and defying him means defying Xemnas, never threatening to report Riku Replica's failure to Xemnas, and then Marluxia openly states Vexen cannot defeat Sora, provoking Vexen into challenging Sora to show his true might, which is exactly what Marluxia wanted as he knew fully well Vexen is no match for Sora.
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 send Axel to eliminate Vexen during his battle with Sora, before Vexen tells Sora of their plan.
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 are 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 is defeated again, and after seeing that Sora is too dangerous and powerful, plans to tell him of Marluxia's plot and, by extension, Roxas's existence. Before he can fully explain, Axel appears and strikes Vexen with one of his chakrams and embeds it into Vexen's body. When Vexen asks why he'd attacked him, Axel explains that he will eliminate Vexen and tells him that he can be "nothing instead of just being a Nobody". Vexen begs to be spared, but Axel replies with a heartless "goodbye" and snaps his fingers, detonating the chakram and incinerating Vexen, making him the first member of Organization XIII to fade out of existence.
As Roxas begins to reclaim more of his memories after entering the computer room of the digital Twilight Town, he remembers the Organization's meeting chamber, with all of the members gathered there, including Vexen.
Despite his recompletion, Even chooses to become Vexen again in order to join the real Organization XIII so he can pursue his research into the Replica Program. Through his work, Organization XIII was able to create enough Replicas in order to host the hearts of its members.
He is later seen in The Caribbean along with Luxord, wishing to study Davy Jones' box and how he managed to remove his heart while still being whole. Luxord asked him why rejoined them, to which Vexen claims that Ansem the Wise never allowed him to pursue his studies as much as he wanted and so he chose to follow Xehanort instead. Luxord told him to leave the World to him and not get involved, to which Vexen obliged. When he found out that the heart inside Davy Jones' box wasn't the kind of heart he thought it was, Vexen left in disappointment.
However, Vexen's true motive becomes clear when he commands a group of Dusks to restrain Ansem, Seeker of Darkness in Twilight Town, in order for Hayner, Olette and Pence to escape with Ansem the Wise. Expressing that he wishes to atone for the past, Vexen then states that he joined the Organization to act as a double agent in order to help Ansem the Wise.
At some point, Vexen became Even again and returned to Radiant Garden.
Vexen 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 made 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.
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 where 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 where he makes two spinning blades of ice and throws them at Sora. In the data battle against Vexen, he has a new attack where 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. Not much else is known about this ability.
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.
While his power still makes him a quite formidable foe for Sora and Riku, with Riku and Sora both being unable to defeat him in their first fight until Sora came out on top in their last fight, although he is one of the most prolific members due to the amount of times he is fought, Vexen, while holding remarkable power, is revealed to be one of the weaker members of Organization XIII alongside Demyx. Thus, while high-ranked, he does not have the power to back his authority, which is why no one in the Organization shows Vexen any respect. Axel 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", while Marluxia was able to easily silence him when he retorted of the failure of his plan and intimidate him by just summoning his weapon. Axel easily wounded Vexen before he could react and destroyed him by specifically burning him with his Chakrams using just a snap of his fingers while Marluxia proved to be able to fight equally with him, although Vexen had notably already been wounded by Sora. In Riku's story, Zexion and Lexaeus is shown to be also superior to him, henceforth making him do their bidding. Most of all is how Vexen deeply fears Xemnas, who as the Superior is doubtlessly effortlessly far much stronger than Vexen, to the point that it seems Vexen would not be able to reason with him if he angers him, as he does not dare to try to meet him in the Chamber of Repose as well as the mere mention of Xemnas being enough to make him back down from challenging Marluxia.
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.