I think it may be the soul eater. My reason for this is that he was the youngest apprentice and apperently was relatively close to Ansem and Xehanort. And since this was Xehanort's weapon than maybe it Zexion uses it. It would explain why the weapon would be kept a secret by nomura. Also, i dont think he is DEAD because the riku replica absorbed him, and the replica had a heart, which went away when riku defeated the replica, and maybe that caused Zexion to get the soul eater that way. Anyway its just a theory, let me know what you think.