He the nobody of Even one of Ansem the wises’s former Apprentices who approaches things like a scientist. He works with Lexaeus and Zexion in order to stop Marluxia and Larxene from overthrowing the Organization He ties to do so by creating a riku replica which he created from the data he extracted from as his weapon to pit against sora but unlike the real riku it has no heart and isn’t afraid to use the darkness.
Later on Vexen confronts Sora after words sora follows him to a town he has never seen before but Vexen says different saying that the futher he goes to rescue Namine the closer he’ll be to becoming Marluxia’s pawn He was Immidiatly killed by Axel for trying to reveal the organization secret and intentions.