This is part of a sub-series tributing the members of Organization XIII, the villianous organization present in Kingdom Hearts II, Chain of Memories, and 358/2 Days.
Saïx is "The Luna Diviner." He is the Nobody of Isa, who had a history with Lea, Axel's somebody. He serves as Xemnas's second-in-command, and there is evidence that he intended to overthrow Xemnas. He appears in Kingdom Hearts II and 358/2 Days.