Lexaeus' original name was Aeleus when he was a lab assistant of Ansem the wise. When he became a Nobody he along with the other five lab assistants became the founding members of Organization XIII. He allies himself with Vexen and Zexion the three of them are known as the underground members. the trio tries devise a plan to counteract Marluxia's plan to overthrow the organization. With Vexen gone and the Riku replica proved itself useless he took matters in his own hands and appears before Riku shortly before Sora confronts Axel and Marluxia on the top floor. Lexaeus tries to convince Riku to join with him but Riku refuses and eliminates him.
After the fight his dying words where that he's finished and says "Zexion forgive me". In Kingdom hearts re-chain of memories in the fight between Riku and Lexeaus. Lexaeus knocked Riku unconscious and then Ansem(Xehanorts heartless) emerges and possesses Riku's body and takes Lexaeus out in one strike.