Saix

How Saix joined the Tourney
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 with his hood down.

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. By the events of the Heartless Invasion at Hollow Bastion, it is revealed that Saïx kidnapped Kairi from Axel. Arriving to interrupt Axel and Sora's conversation, the former fleeing, Saïx ensures that Axel will be rightly punished while playing with Sora's feelings over Kairi by having get onto his knees and beg at the Nobody's feet.

Seeing Sora's devotion, Saïx sees this as a further opportunity to enrage and confuse Sora by eliciting the boy's fury upon refusing to take him to Kairi. Conjuring Heartless, Saïx reveals the Organization's goals to have the hearts held within Emblem Heartless be released by the Keyblade, so that they may flock to Xemnas's synthetic Kingdom Hearts and complete. Despite Maleficent's interference, Saïx has the Dusks he summon subdue her so he can force Sora into defeating more Heartless and release more hearts. But before he can finish his ploy, Maleficent conjures a portal of darkness to spirit Sora and his group away.

Later, as Sora makes his way to the Castle That Never Was, Saïx visits Kairi in her cell while making her feel guilty by revealing that she is the "fire that feeds Sora's anger." However, prior to fading back into Kairi, Naminé breaks her original self from her cell and attempts to take her to safety. But Saïx attempts to block the girls' escape before 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.

Welcoming Sora to the castle, Saïx looks lovingly upon Kingdom Hearts while telling Sora that Kairi no longer needs him and is with a "friend from the darkness". Saïx also states that the Organization's Kingdom Hearts is nearly done, summoning more Heartless for Sora to destroy. Though he notices that Maleficent has also infiltrated the castle along with Pete, Saïx takes this in stride as he vanishes see Xemnas. Once having permission to kill Sora as Xemnas deems their Kingdom Hearts complete, Saïx smiles savagely and remarks that he's waited so long to hear that.

Awaiting Sora and group in Addled Impasse, Saïx 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 Lunatic claymore, he creates a potent shockwave that pushes Kairi and Riku back towards the entrance of the room as he quickly erects a force field to Sora, Donald, and Goofy for the final time while using the light of Kingdom Hearts to become a berserker. However, despite the incredible power and abilities Saïx 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.

Months after his death, he was reawakened by a curious boy named Jeff. Saix decides to fake his friendship towards Jeff in order to claim his heart.

When highlighted
Saix holds his Claymore sword Lunatic behind him.

After the announcer calls his name
Saix rages then swings Lunatic then throws it as the camera zooms saying "Save your remorse."

Anger Shockwave (Neutral)
Saix swings Lunatic down, sending a shockwave of energy at the opponent.

Crazy Sword (Side)
Saix throws a claymore at the opponent, releasing energy on impact.

Rage Split (Up)
Saix jumps into the air roll slashing with Lunatic.

furious (Down)
Saix spins forward doing four slashes.

Berserk (Hyper Smash)
Saix shouts "Moon, shine down!" then goes Berserk. In this state, his attacks become stronger, his defense is more durable and his speed is increased. This lasts 70 seconds.

Wrath (Final Smash)
Saix goes enraged saying "Don't vex me." then rush across the battlefield, causing shockwaves. Pressing A during this will cause Saïx to jump into the air and release an area burst shockwave, inflicting massive damage to nearby targets. He stops after rushing for eighteen seconds.

Bonus Costume
Saix holds possession of one bonus Costume, that being his original form, Isa. To unlock, one must complete Survival Mode with Saix. After the final battle with Big-Zam, the announcer will say "You've earned a new costume!" and a Smash Ball (appears on the lower-right corner) will appear with a message saying "You've opened Isa, Saix's old form. There may still be some friendship for him and Lea yet!" Then, highlight saix and press Minus.

Victory Aniamtions

 * 1) Saix throws Lunatic up, then catches it and slams it down saying "Grovel before me!"
 * 2) Saix does two slashes, then a kick, then plants Lunatic in the ground saying "I want to see you die fighting."
 * 3) Saix flashes moonlights off Lunatic then pierces the claymore and says "All shall be lost to you!"

On-Screen Appearance
Saix appears out of the ground and takes out Lunatic saying "Do you feel it, the moon's power?"

Trivia

 * Saix's rival is one of the Barney children who first appeared in Let's Build Together, Jeff.
 * Saix shares his English voice actor with Xiahou Dun, Meng Huo, Kisame Hoshigaki, the Tyrannosaurus Rex, Darknut, Jinpachi Mishima, Ujutsu Mujuro, Toxicroak, Ninjask, Rasetsu, Grant Danasty, Arbok, Crocomire, Rai-Dei the Blade, Shadow the Hedgehog, Greenfist, Bonechill and Potemkin.
 * Saix shares his German voice actor with Magnezone and Dimitri Petrenko.
 * Saix shares his Arabic voice actor with Shiden, Takamaru, Cornell, Sektor, Guardian and Rain.
 * Saix shares his Mandarin Chinese voice actor with Hayato Jin (in Getter-2) and Mr. Ryan.