Saïx (サイクス, Saikusu?, also spelled without diaeresis as Saix), also known as the "Luna Diviner" (月に舞う魔人, Tsuki ni Mau Majin?, lit. "Devil Dancing in the Moon"), is a Nobody with an X-shaped facial scar who can assume a berseker state of mind by using moonlight. Wielding a claymore called "Lunatic" (ルナティック, Runatikku?), he commands the Beserker Nobodies who wield weapons similar to his.[7] His original self was named Isa (アイザ, Aiza?),[39] and used to be best friends with Lea, Axel's original self.[40] Being in charge of relaying Xemnas' orders to the other members, Saïx joined the Organization around the same time as Axel. However, lacking the care-free personality and morals his original self had, Saïx used Axel to become Xemnas' second in command, and later feels betrayed when Axel befriends Roxas and puts his plan to get leverage over Xemnas on hold. Saïx appears completely void of emotion and completely embraces his status as a Nobody, yet continually reflects on previously having a heart. The character's Japanese voice actor is Ginpei Sato, while the English voice is provided by Kirk Thornton.[11]

During Kingdom Hearts: 358/2 Days, Saïx assigns missions to the other Organization members. He is also assigned by Xemnas to oversee the progress of Xion. Late in the game, he attacks Roxas when he attempts to leave the Organization, only to lose to the younger Nobody. Later in Kingdom Hearts II, after appearing at Hollow Bastion along with the other members, Saïx appears in Twilight Town after Sora defeats his Berserker Nobodies and warns Sora of Axel's intention of turning him into a Heartless with intentions to stirring unrest in the boy. After Sora defeats the Heartless army at Hollow Bastion, Saïx appears and explains Sora's purpose in the Organization's plans and refuses Sora's request to meet Kairi, whom Saïx kidnapped prior from Axel after Xemnas gives him orders to. After Saïx summons a horde of Nobodies, Maleficent attempts to use her own Heartless to battle Saïx, but his Nobodies quickly dispatch them and overwhelm Maleficent. On his climb through the Castle That Never Was, Sora confronts Saïx. Saïx fights Sora while using the moonlight from Kingdom Hearts to assume a beserker state, but Sora defeats him and he fades into darkness while pining for Kingdom Hearts to give him his heart back.[7]