Number 6 of Organization XIII
Also known as the "Shadow Walking Tactician"
Like Axel, Zexion, the "Shadow Walking Tactician", is a skilled manipulator by nature, though as we discover, not skilled enough. Appearing in Chain of Memories, his talents lay outside of battle, even to the point where he does not possess a weapon; he is the only Organization member who does not fight in a gameplay battle and lacks a voice actor.
He is one of two Organization members who appear only in Riku's story. He has the ability to identify individuals by their scent, sensing when they die, and seems to be able to do so from great distances. His element is shadow.
Zexion informs Vexen and Lexaeus when he senses two new scents in the lowest basement of Castle Oblivion. One of the scents is Maleficent, and he senses that she is eliminated by the other visitor, one with a scent "exactly similar" to that of the Superior. With little information on what was happening, he and the others decide to await development. Not long after, he identifies the other guest in the castle: Riku. He observes the boy's progress through Castle Oblivion until Vexen's death, when he and Lexaeus decide that they should use Riku just as they believe Marluxia will use Sora.
After Larxene, Lexaeus and even Marluxia are killed, Zexion sees that Riku is no longer needed and attempts to destroy him himself. He leads Riku into a manifestation of Destiny Islands, Sora and Riku's home, in which Riku sees his friends disappear. Zexion appeares and wages psychological war on Riku, telling him about how he threw away his friends and his home, opened the door to darkness, and destroyed his islands. Following Riku's battle with a Darkside Heartless, Zexion uses his power to disguise himself as Sora and attacks him, telling him that he's become nothing more than a pawn of the darkness. He engulfs Riku in light, but a vision of Kairi manages to convince Riku to shed his fear of the darkness. With newfound strength, Riku follows Zexion's scent through the light and strikes him, but Zexion escapes. His escape was short-lived; Axel brings the Riku Replica down to him and convinces the replica that he can become real by gaining power that the real Riku does not have. The replica agrees, and with Axel's suggestion, strikes down Zexion without difficulty.
He was originally Ansem's youngest lab assistant and test subject, under the name of Ienzo. He was the one that convinced Ansem, with his passionate words, that they should expand the horizons of their research.
Kingdom Hearts II Alpha Ultimania, a short guidebook released to promote the game, lists him as being "fallen into ruin", though the meaning of this remains unclear.
In the Chain of Memories manga, Zexion spends most of his time fooling around with various puzzles.
Zexion, much like the other characters from Chain of Memories, are only included in one cutscene and a journal entry in Kingdom Hearts II.