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. He is Rank VII within the original Organization XIII and a member of the true Organization XIII. 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.
In the original Organization, Saïx serves as the second-in-command to Xemnas, and is a major antagonist in Kingdom Hearts II and Kingdom Hearts 358/2 Days. Fittingly, Saïx is the second-to-last member of the first Organization to fall. Along with all other original Organization members in Kingdom Hearts 358/2 Days, Saïx is a playable character in Mission Mode.
 Kingdom Hearts 358/2 Days
Saïx 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. Saïx is also revealed to be the one who picked which members would be going to Castle Oblivion, namely the members suspected of being traitors, as well as anyone else who was "getting in the way".
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 of them 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.
 Kingdom Hearts 3D: Dream Drop Distance
Despite his previous destruction, Saïx is used by Young Xehanort to serve as one of the thirteen vessels for Master Xehanort's heart. When Xehanort's attempt to make Sora his final vessel ends in failure due to Lea's intervention, Saïx attacks his former friend. As their weapons collide, his hood is blown off and his identity is revealed to Lea, shocking him. Lea knocks Saïx away to regroup with Riku and King Mickey. After Donald and Goofy arrive, Saïx and the rest of the true Organization XIII return to their own time periods to await the time to properly forge the χ-blade.
Saïx wears a basic Organization black coat, along with the usual black gloves and boots. He has long blue hair vaguely reminiscent of Vexen's in style and length, though Saïx's is a bit wilder at the top. Most of his hair is tucked into his cloak, though a portion is not and falls over the back. Saïx also shares a few traits with fellow Organization members, Xigbar and Xaldin, sharing his pointed ears and gold eyes with the former and his stud earrings with the latter which he maintains from his original persona. Another distinctive trait of Saïx's is the X-shaped scar on his face. The vertex of this "X" is between his eyes and it extends to about halfway up his forehead in one direction and to just under his eyes in the other. When Saïx enters his "Berserk" state, his eyes turn completely yellow, his scar becomes more jagged and a bit larger, and his hair becomes much messier.
Saïx's transformation under the moon's power is a possible reference to several things, including: the ancient belief that the moon caused madness, hence the term "lunatic"; the modern belief that the phases of the moon can affect a person's mood; the legend of the werewolf, humans who became savage beasts under the influence of a full moon; as well as the berserker warriors of the Norse, who wore clothes made from bear skins as they went into battle in an insane fury and are the source of the modern term "berserk."
In Kingdom Hearts Birth by Sleep, Saïx's hooded body model is used for Young Xehanort.
Next to Xemnas, Saïx appears to be one of the most "Nobody-like" of the Organization. Despite claiming to remember what possessing a heart was like, he behaves mostly as a cold, emotionless, and efficient operative until his final battle. He is very aware of how emotions are unreal to them, unlike other members who imitate their emotions. However, in Kingdom Hearts 358/2 Days, it seems he values his friendship with Axel from their time as humans over that which Axel has formed with Xion and Roxas—the former friendship confirmed in the Secret Reports. This possibly indicates he believes that Nobodies cannot even try to genuinely show emotion or form a real friendship without having had one with another beforehand, thus having memories to fall back on; and he does in fact see Xion as nothing more than a mere puppet. This friendship between them weakened due to how Saïx changed too much after becoming a Nobody, according to Axel, leading to Axel's ignoring him for Roxas and Xion. Thus, Saïx has a colder demeanor towards Axel in the second game.
In almost all of his appearances, he acts very calmly, never losing composure in even the direst of situations. The only thing Saïx seems to "care" about is acquiring a heart and, by extension, Kingdom Hearts. More than any other member of the Organization, Saïx is portrayed the most dedicated to their goal, repeatedly stating his desire to "truly, finally exist". Even his last words were to Kingdom Hearts, sadly asking for his heart. Though this doesn't make him blind to Xemnas' plans to use Kingdom Hearts for more power, as he had originally plotted with Axel to overthrow Xemnas—not for the same goals as Marluxia, but to regain their hearts. Saïx can also be quite cruel, claiming to know very well how to injure a heart. He had a keen insight into manipulating others, such as when he toyed with Sora's emotions by having him beg to see Kairi, only to refuse his request. Also proof of his manipulation skills is how he sent the "useless" members to Castle Oblivion under the guise of Xemnas's orders to clear the way for his and Axel's plans, moving up the ranks while eliminating any competition or rebellion.
However, his usual calm exterior belies a more savage nature. In his final confrontation with Sora under the heart-shaped moon of Kingdom Hearts, he becomes a berserk force of destruction. Unlike his calm and reserved manner, in battle, he roars, shouts, and attacks relentlessly with no sense of self-control and portrays a psychotic behavior under the influence of Kingdom Hearts.
Saïx is a powerful member of Organization XIII, and a capable fighter. Although he appears calm and collected throughout when speaking, in battle he becomes a wild, dangerous opponent. He uses brute strength and his large Claymore to attack, but his true danger is his "Berserk" form, which uses the power of the moon. A "Berserk" gauge in the top of the screen measures how close he is to becoming berserk. When it fills, Saïx's Lunatic Claymore shifts shape and produces a white aura which serves as extra blades. Saïx himself grows bestial, with glowing eyes, manic hair, and bared fangs. Saïx's strength and speed increase greatly, and he will rapidly slam his Claymore into the ground, producing shockwaves. When he swings or throws the Claymore, it will leave a wake of blue flames.
However, Sora can pick up any of the Claymores that Saïx releases, allowing him to use Reaction Commands that will knock Saïx out of his berserk state, as well as inflicting significant damage. When knocked out of his berserk state, Saïx will spend most of his time with his arms stretched towards the moon, absorbing its power.
The Nobodies under his command, the Berserkers, follow his attack style, using brute strength and a large weapon, but share the same weakness as Saïx.
In Kingdom Hearts 358/2 Days, Saïx's Limit Break is also called Berserk. During Berserk, Saïx goes into a berserk stage; he rushes across the field, dealing continuous, devastating damage to enemies that he comes down hitting with his claymore. When the attack button is pressed, he will jump and perform a stronger smashing attack. Each smashing attack is followed up by a shockwave.
During Saïx's Final Limit, Saïx goes into an even more berserk stage; He comes down to attack even more often, and exerts a sphere of his strength straight from himself while in the air before coming down. The shockwaves that come after each attack also increase in range.
Saïx wields a massive blade known as a Claymore. His signature Claymore is known as Lunatic. Despite many dissimilarities to real claymores, Saïx does use a very large, two-handed bladed weapon. His Claymore is decorated with many additives that resemble the Nobody logo, in blues and yellows. When in normal form, Saïx attacks with powerful slashes and combos with the bladed edges.
Although it seems to be a completely different weapon, the Cursed Hammer of the Berserker Nobodies is used in much the same way, and even has the same Reaction Command.
The names of Saïx's various claymores are all references to astronomy or astrology (especially the moon), with the exceptions of Crocea Mors (Latin for "yellow death") in the Japanese version, which is the purported name of Julius Caesar's sword; and Just Desserts, his joke weapon.
 See also