Saïx, or Saix (, Saikusu?) in the Japanese version, the "Luna Diviner", has an X-shaped scar on his face and wields a gigantic claymore. He controls the element of moon and the Berserker Nobodies, who wield weapons much like his own. His powers and fighting style are a play on the lunar effect and he is somewhat psychotic in nature.

Despite his rank, Saïx is respected as if he was the second in command. He is a calm and collected man who shows no emotion, belying his extensively cruel nature. Axel has a clear dislike of him. Saïx becomes the one to brand Axel as a traitor to the group for attempting to derail the Organization's plans for Sora. It was Saïx who caught Kairi after she got away from Axel and decided to use this to his advantage. After Sora defeated the Heartless army at Hollow Bastion, Saïx appears and summons even more Heartless for Sora to defeat after he fills Sora's heart with anger when he refused to bring him to Kairi. Later at the World That Never Was when Naminé helped Kairi escape, Saïx appeared to deal with the two, but was stopped by Riku in the guise of Xehanort's Heartless, but Saïx escaped before any permanent damage could be done. Despite appearing calm, Saïx shows his true beastlike nature when he fights Sora in the World That Never Was, going into a berserk state by absorbing the moonlight from Kingdom Hearts. However, he was mortally wounded and died while pining for Kingdom Hearts to give him his heart back.