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Organization XIII debuted in Kingdom Hearts: Chain of Memories, then known as simply, "the Organization." In Sora's story, the player was introduced to (in no particular order) Marluxia, Axel, Larxene, and Vexen. In Reverse/Rebirth, Zexion and Lexaeus were added to the cast.
The Organization also appeared in Kingdom Hearts II. In this second installment, the player meets (once again, in no particular order) Xigbar, Xaldin, Saïx, Demyx, Roxas, the leader Xemnas, and the returning member, Axel.