Majority of information taken from The Keyhole-Saïx.
Saïx, the Luna Diviner (月に舞う魔人, Tsuki ni Mau Majin, lit. Demoniac Dancing in the Moon), is the Nobody of Isa, Rank VII within Organization XIII, as well as the second-in-command, and next to Xemnas, the main antagonist in Kingdom Hearts II and Kingdom Hearts 358/2 Days. He derives power from the Moon, and uses this power to enter a berserk rage where he mercilessly attacks opponents with his massive claymore. He commands the Berserker Nobodies.
Fittingly, Saïx is the second-to-last member of the Organization to fall. Along with all other Organization members in Kingdom Hearts 358/2 Days, Saïx is a playable character in Mission Mode.
Kingdom Hearts Birth by Sleep
Main article: Isa
Kingdom Hearts 358/2 Days
Saïx plays a relatively important role in Kingdom Hearts 358/2 Days. He assigns Roxas his missions in The Grey Area at the Castle That Never Was, and is present when Xion joins the Organization. He remains in this station throughout the beginning and middle of the game.
Saïx is the one who gave Xemnas's orders to Axel to rid Organization XIII of the traitors in Castle Oblivion. It is generally accepted by the remaining members that at this point the two planned to not only eliminate traitors, but also anyone who got in the way of their plan, such as Vexen and Zexion.
After Axel's return from Castle Oblivion and the growing friendship between Axel, Roxas and Xion, a rift between Saïx and Axel begins to form. He eventually confronts Axel, revealing the Nobody's past, along with his true name. By the time of Roxas's departure from the Organization, Saïx dismisses his previous plans with Axel as "just an idle fantasy", as both have changed.
Working on Organization XIII's plans for Sora with Xemnas, he later challenges Roxas to a battle of will when the Keyblade wielder attempts to leave Organization XIII. He loses the battle, and Roxas escapes. Up until Xion's destruction and absorption at the hands of Roxas, Saïx views the Sora copy as nothing but a mere puppet. He even insists that the others would say "it" when referring to Xion, rather than "she".
After Xion's end, Xemnas orders Saïx to retrieve Roxas, who is on his way to the Castle That Never Was to defeat Organization XIII and free Kingdom Hearts, because he believes it will bring Xion back. While not confronting Roxas himself, it is possible that Saïx was the one who sent the Neoshadows after Roxas, who destroyed the Heartless easily.
Kingdom Hearts II
Saïx's first appearance is in Hollow Bastion, along with the surviving Organization members, excluding Axel, to taunt Sora. He remarks "You're doing well", but like the others (except for Xigbar), does not confront Sora, and simply stands next to Xemnas.
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 declares he is not worried, prompting Saïx to remark that the Organization would want nothing to happen to Sora. He also cryptically remarks that the Organization knows how to "injure a heart". He then opens a portal of darkness 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 to report to Xemnas that he succeeded in placing Sora in a state of confusion and frustration to keep him going toward their goal. Saïx also states that Axel may prove a thorn in their side, which leads to Xemnas giving him orders to deal with the renegade.
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 is not mollified, his only goal being to enter the realm of darkness to rescue Kairi. His face unreadable, Saïx asks Sora to prove just how much he cares about Kairi. In response, Sora actually gets onto his knees and begs at the Nobody's feet. Seeing Sora's devotion to the Princess of Heart, Saïx sees this as a further opportunity to enrage and confuse Sora so that he might further the Organization's goal. He refuses Sora's wish, eliciting a hot fury. Bending Sora's anger to suit his own needs, Saïx convinces Sora to destroy more Heartless so that the hearts held within them are released by the Keyblade, so that they may flock to Xemnas' synthetic Kingdom Hearts, which will be all the closer to completion because of it.
At that moment, Maleficent appears, declaring with much aplomb 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 places herself between Sora and the Nobodies, telling him to devise a way to vanquish the Organization. Swarmed by Nobodies, she is overwhelmed and disappears, allowing Saïx to resume convincing Sora to release more hearts. But before he can finish his ploy, Sora, Donald, and Goofy are rescued by a portal of darkness that swallows them up (which was Maleficent's doing).
Saïx is seen again talking to Kairi in her cell in the Castle That Never Was. He tells her, much to her horror, that she is the "fire that feeds Sora's anger." Kairi realizes that Sora is getting lured into a trap all because of her. However, Naminé unexpectedly appears in her cell, and rescues her through a portal of darkness. The two halves then begin fleeing the castle to try and make good their escape. Saïx soon appears and attempts to stop Kairi and Naminé, but Riku, still cloaked and in the form of Ansem, Seeker of Darkness, attacks him from behind. Saïx does a back flip in the air, and lands a few feet away. Riku slams him up against the wall, intending to finish him once and for all. However, Saïx opens up a portal behind him and sinks away into the wall with a smirk.
Saïx again appears in the Castle That Never Was to Sora, who had managed to work his way through the Organization's floating fortress with the help of Donald, Goofy, Riku, and Kairi. He looks lovingly upon his Kingdom Hearts, his source of power. He tells Sora 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 the Organization's Kingdom Hearts is nearly done, pointing to the heart-shaped moon in the sky. He then enigmatically asks Sora if he can hear the euphoria of the countless hearts he has harvested. He 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, wondering in an amused voice if the intrusions will ever end, 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 this "charade," and Xemnas grants him permission. Sora's part in their plan is done, he has given them every heart required, and he can now safely be killed. Saïx smiles savagely and remarks that he's waited so long to hear that.
Elsewhere in the castle, after Luxord's defeat, Saïx awaits Sora, Donald, Goofy, Riku, and Kairi in a room with a direct view of Kingdom Hearts: Addled Impasse. He calls Sora by the name of "Roxas", saying only he could have made it this far, though Donald vehemently states that he is Sora. Saïx seems not to care, simply stating: "Different name, same fate." Then, summoning his claymore, he creates a potent shockwave with sheer strength alone. This power manages to push away Kairi and Riku back towards the entrance of the room. He then quickly erects a force field, separating them from the battle, yet doing no harm. Them taken care of, he confronts Sora, Donald, and Goofy for the final time. He finally lets his outer decorum slip away. Under the influence of the moon, he becomes savage and unrestrained, truly a model for his Berserker Nobodies.
However, despite the incredible power and abilities he drew from the moon of Kingdom Hearts, he is overcome. Defeated and mortally wounded, he limps away from Sora to the ledge overlooking the moon. With the last of his strength, he asks: "Why...Kingdom Hearts...Where is my heart?" All his power gone, he now fades back into darkness. Sora watches with a brief look of pity and regret, then turns to rejoin Kairi and Riku.
Day 9: On Track
Xion is being monitored. Roxas, meanwhile, has begun his training exercises. It is imperative we keep Xion from having undue contact with Roxas until we are certain it will perform as expected - which will take another 10 days, if Vexen's convictions are to be trusted. As for the Keyblade Master, Marluxia will no doubt be keeping an eye on him.
Day 51: Measures Must Be Taken
The Replica Program continues apace, but something has gone wrong at Castle Oblivion. The Dusks returned with word that none of those we dispatched survived - but the news came in fragments. I still know too little to extract the truth. I doubt Axel would be among those lost, but I must plan for the worst. First, I must locate the Keyblade master.
Day 94: All Goes Apace
The hearts collected by our two Keyblade wielders, Roxas and Xion, have gathered together; and now, almighty Kingdom Hearts waxes large in the night sky. Our efforts have come to bear fruit, nearly ripe for the plucking. All plans proceed smoothly - alarmingly so, in fact, though this is no time to be deterred by paranoia.
Day 150: Dealing with Xion
As expected, the Duplicate is starting to show its limits. The Program showed promise, but a puppet is just a puppet: something to be toyed with until it breaks. I am utterly at a loss as to what Roxas and Axel see in that thing. How best to dispose of it merits my consideration going forward.
Day 171: The No. i Project
Xion failed to complete its mission. If this continues, destroying it and using the next Replica as the Duplicate would undoubtedly yield a higher-grade copy. No. i was among the initial lot, which naturally raises questions about its capabilities. At present, it is nothing short of broken. I cannot fathom why Xemnas would want to keep it.
Day 174: Just a Replica
Xion continues to sleep. All analysis on its function suggests no marked change. The copying appears to be working, but a Replica is a Replica. My latest report to Xemnas on this net me no clear answers. He simply stood, smiling. At times I find his thinking impenetrable.
Day 277: True Agenda
Axel and Roxas are up to something. No plot they concoct will change the fact that the Organization has cast off Xion, though. Our plans have split, diverged, and advance now to the next stage. The No. i project and Replica Program merely paved the way. Our true goals lie elsewhere.
Day 298: Change of Plans
Axel failed to report that the impostor and Xion are working together. Instead, that news came from Roxas. The boy is far more easily handled than Axel, but now Axel has captured Xion and returned her to us. His motives are impossible to read. Our plans can be altered if necessary, but doing so can only delay their realization.
Day 356: Unforeseen Events
Our plans never accounted for the possibility of both Xion and Roxas leaving. How did this come to pass? When did Roxas grow strong enough to outmuscle me? What were you really after, Lea? We joined the Organization at the same time, and formulated our plan. At this point, it's just an idle fantasy. Everything changed. You, and me.
- For more pictures and screenshots of Saix, click here.