Vexen

Vexen, the Chilly Academic, is number IV in Organization XIII and is one of its founding members. Despite his high rank, he is the first Organization member to be eliminated. He also created the Riku replica from the real Riku's data.

Vexen carries a shield into battle, and fights using ice magic.

Role in Castle Oblivion
In Kingdom Hearts:Chain of Memories, Vexen had parts in both Sora's and Riku's storylines. During the internal struggle in the Organization involving two younger members, Larxene and Marluxia, rebelling against the senior members, Vexen acted as a double agent, aided by the Riku replica. After engaging Riku in a cautious battle, Vexen used the data he obtained from the fighting to create a copy of Riku that lacked a heart. He offered the use of the replica to Marluxia, who used Naminé, a girl with mysterious power over memory, to implant false memories into it and send it questing for a heart. The younger members were also able to use the Riku replica to make Sora compete for the right to save Naminé by having the replica attack him periodically, seemingly on her behalf.

Involvement with Sora
Vexen fought Sora as well, using his shield and the power of ice, but was originally defeated. Marluxia coerced him into fighting Sora again after they lost control of the replica, and Marluxia, using the authority granted to him as lord of Castle Oblivion despite his low rank, pronounced the experiment a failure. He then ordered Vexen to eliminate Sora, threatening to tell "the Superior" of his failure if he refused. Vexen agreed and confronted Sora in a Twilight Town room of the castle.

The two clashed in front of the gates to the mansion, and once again, Sora was victorious. Vexen, weakened from the fight, made to tell Sora of Marluxia's plan to manipulate him. This would have revealed that Naminé was a Nobody manipulating Sora's memories, which would have made him aware of the dark realm and eventually of Roxas. Before he could fully explain, however, he was struck from behind by Axel, another cohort of Larxene and Marluxia. Vexen begged to be spared, but Axel ignored him and cut him down, making Vexen the first of the Organization to die.

Vexen's death allowed Axel a great advantage in his own plot. Marluxia had sent Axel to assassinate Vexen not only to silence him, but to test Axel as well. Vexen's death allowed Axel to gain Marluxia's full trust, which later granted him an opportunity to double-cross them by freeing Naminé from Marluxia's control.

Origins
Vexen is the nobody of Even, who was one of the top apprentices of Ansem the Wise. He is mentioned in the Secret Ansem Reports as early as the first psychological tests on the head apprentice Xehanort as being particularly interested in memory. He helped found the core of the Organization, along with Xehanort, Ienzo, Bleig, Dilin, and Aeleus. Unlike many of them, he retained his scientific nature, approaching many problems experimentally. He is a calculating and scientific character by nature, even able to use data from a fight to program the Riku replica for combat. This scientific brilliance and his power over ice earned him the title the Chilly Academic.

Videos
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Vexen vs. Riku

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Vexen vs. Sora