Vexen (ヴィクセン, Vikusen?) is a Nobody known as the "Chilly Academic" (いてつく学究, Itetsuku Gakkyū?). Vexen's original self was a scientist named Even (エヴェン?), who met Ven and felt they would cross paths someday. His later interest in studying Xehanort"s memories resulted in him becoming a Nobody. Vexen can control ice and carries a large shield called "Frozen Pride" (フリーズプライド, Furīzu Puraido?, lit. "Freeze Pride") in battle.[7] The character first appeared in "Kingdom Hearts: Chain of Memories", where Tatsuya Kando provided the voice for simple battle expressions.[23] Vexen was included as an optional boss in the Japanese exclusive "Kingdom Hearts II Final Mix", and was voiced by Nachi Nozawa.[24] Nozawa returned to provide voice acting for dialogue in the re-release of "Chain of Memories" and "Kingdom Hearts 358/2 Days".[13] Vexen's English voice acting was done by Derek Stephen Prince.[25]

As revealed in "Kingdom Hearts 358/2 Days", training Roxas before leaving for Castle Oblivion, Vexen has been experimenting with the creation of Replicas, the first of which is Xion. In "Chain of Memories", Vexen sides with Zexion and Lexaeus, other members occupying Castle Oblivion's basements, to prevent Marluxia's plan to overthrow the Organization.[26] He battles Riku in order to gather data to create a Replica of him, and is later assigned to investigate Marluxia's intentions.[27][28] Vexen allows Naminé to manipulate the Riku Replica's memories to test Sora's worth.[29] When the Replica fails to defeat Sora, Marluxia threatens to reveal the shortcoming to Xemnas, forcing Vexen to fight Sora himself in an attempt to kill the boy. After their battle, Vexen gives Sora the card to access Twilight Town in Castle Oblivion with the intention of revealing Roxas's existence.[30] As a result, Marluxia considers him a traitor and sends Axel to kill him.[31] In Twilight Town, Vexen attempts to reveal to Sora why Twilight Town feels familiar to him.[32] Vexen battles Sora again, but is struck down and incinerated by Axel as he is about to reveal Marluxia's plot.[33] Though Vexen's death was unintended, putting the Organization at a disadvantage, Axel did it to ensure his research isn't used by Marluxia, in addition to gaining his and Larxene's trust.[34]