Vexen

Vexen, the Chilly Academic (いてつく学究, Itetsuku Gakkyū?, lit. Freezing Scholar), is the Nobody of Even and is Rank IV within 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. He appears in Kingdom Hearts 358/2 Days as a playable character in Mission Mode.

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. Adviser 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 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'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.

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, 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 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.

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 Sora 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.