Saïx, the Luna Diviner, is number VII in the Organization and the last of Xemnas's companions to fall to Sora, as well as the apparent second-in-command. He uses the element of moon, utilizes a claymore in battle, and controls the Beserker Nobodies.
Hollow Bastion
Saïx first appears in Hollow Bastion, along with all surviving Organization members sans Axel, to taunt Sora. He remarks "You're doing well", but like the others, does not confront Sora (save for Xigbar).
Twilight Town
Upon Sora's second visit to Twilight Town, the trio fight off several Nobodies that were attacking Seifer and his gang. After they save him, Saïx appears before them, applauding their battle skills. He warns them of a traitor in the Organization: Axel, and he says that he might be in Twilight Town somewhere. Sora insults the Organization, remarking on how they're not a "very organized Organization". Saïx is unfazed, and tells Sora to watch his back, for Axel will stop at nothing to turn him into a Heartless. Sora is not worried, leading Saïx to remark that the Organization would want nothing to happen to Sora, also saying they know how to "injure a heart". He then opens a portal behind him, and Sora thinks of jumping in after him, though Saïx overhears this and advises not to do so, lest he end up like Riku. Sora inquires as to what he means, but Saïx immediately leaves.
The Heartless Invasion
Saïx next appears in Hollow Bastion during the Heartless invasion, interrupting Axel and Sora's conversation, and forcing the traitor Nobody to flee. Saïx ensures that Axel will be rightly punished, but Sora only wants to enter the realm of darkness to rescue Kairi. Saïx asks Sora to prove just how much he cares about Kairi, and prove it he does, as he actually gets onto his knees and begs. Seeing Sora's devotion to the Princess of Heart, Saïx refuses Sora's wish, eliciting rage from the Keyblade master. Saïx only uses this to his advantage, ordering Sora to destroy more Heartless, so that the hearts held within them are released by the Keyblade. Then they may flock to Kingdom Hearts, which the Nobodies can use to truly exist once it is finished.
At that moment, Maleficent appears, declaring that Kingdom Hearts will belong to her. She summons more Heartless to attack Saïx, despite Sora's protests, but Saïx calmly summons several Nobodies to destroy the attacking Heartless. Maleficent then puts herself between Sora and the Nobodies, telling him to devise a way to vanquish the Organization. Soon, she is overwhelmed and disappears, allowing Saïx to resume having Sora release more hearts. But before that could happen, Sora, Donald, and Goofy are rescued by a portal of darkness that swallows them up.
The Final Battle
Saïx again appears in the World That Never Was to Sora, who had managed to work their way through the Organization's floating fortress. The Organization member says that Kairi no longer needs him and makes a cryptic remark about a "friend from the darkness". Sora is not deterred by Saïx's manipulative comments, but Saïx is not concerned. He tells Sora that Kingdom Hearts is nearly done, pointing to a heart-shaped moon in the sky. He then summons more Heartless for Sora to destroy, but his attention is quickly turned to Maleficent, who has also infiltrated the castle along with Pete. He takes this in stride, and soon after vanishes to check on Kingdom Hearts.
Saïx appears to Xemnas, who is watching Kingdom Hearts come closer to completion. He asks the Superior if he can end the charade, and Xemnas allows him to do so. Saïx smiles and remarks how he waited so long to hear that.
Saïx awaits Sora, Donald, Goofy, Riku, and Kairi in a room with a direct view of Kingdom Hearts. He calls Sora by the name of "Roxas", saying only he could've made it this far, though Donald vehemently states that he is Sora. Saïx does not care, and summons his weapon, simply says "Different name, same fate", and creates a powerful shockwave with sheer strength alone. The attack does no harm, but it manages to push away Kairi and Riku, separating them from the battle. He then confronts Sora, Donald, and Goofy, at last revealing the savage nature that lay hidden under his calm exterior.
However, despite Saïx's great strength and skill, he loses the battle. Defeated and mortally wounded, he slowly steps towards the moon. He almost sadly asks Kingdom Hearts where his heart is, before fading back into darkness.
Personality
Like most members of the Organization, Saïx is cold and emotionless. In almost all of his apperances, he acts very calmly, never losing composure in even the direst of situations.
But his calm appearance belies his true self. In his final confrontation with Sora under the heart-shaped moon of Kingdom Hearts, he becomes berserk. Unlike his calm and reserved manner that is usual of him, in battle he roars, shouts, and attacks like a ferocious beast that loses all self control. In fact, he is practically psychotic with the influence of the moon.
Fighting Style
Saïx is a very powerful member of the Organization, possibly second only to Xemnas. Throughout most of the game, he is seen as a calm and collected figure, but in battle, he is a wild, unstoppable beast. True to his use of brute strength, he employs a claymore, using it to draw upon the power of the moon,. When he does this, he temporarily gains a dramatic increase in power, becoming nigh invincible. His attacks form blue flames, creating blazing paths and walls, as well as gigantic shockwaves, only adding to his already tremendous brute strength. He can also hurl his claymores at his opponents, and instead of retrieving it, easily forms new ones. However, this allows opponents to use the old claymores against him, which when used properly, can cancel out Saïx's powered form.
The Nobodies under his command, the Beserkers, follow his attack style, using brute strength and a large weapon, which have the same weakness Sora exploited when facing Saïx.