This was at one point the room of Organization XIII's most dextrous member; number IV, Vexen. He was the nobody of Ansem's apprentice Even, and his room shows through more than any of the others. Equations, lab equipment, and multiple specimens crowd his living quarters to the point where one can hardly move around. On his wall is a framed diploma from Tokyo University.

Vexen was stationed at Castle Oblivion, and dwelt in the thirteen basement floors with Lexaeus and Zexion. He had a slight superiority complex over those who had a number higher than his, and thusly acted as the trio's unofficial leader. He saw no value in Sora's existence, but decided to observe him regardless in order to foil Marluxia's ambitions.

While battling Riku, Vexen collected data, which he would later use to create a Riku replica. This replica, with Vexen pulling its strings in a sense, would harass Sora as he climbed the castle floors. After a dual with Sora, Vexen gave the boy a card which allowed access to Twilight Town. This act made Vexen a traitor in Marluxia's eyes, and he orderd Axel to "eliminate the traitor". Vexen met his end in Twilight Town when Axel blew him up.