Saïx makes an appearance in 3 Kingdom Hearts games so far.
Kingdom Hearts II
Saïx makes his first appearance in the Baily at Hollow Bastion. His identity is not clear yet and he is with the rest of Organization 13, laughing. Next appearance, Saix shows up to warn Sora about Axel's intentions after Sora finishes off his Berserkers in Twilight Town. Before the bazillion Heartless battle at Hollow Bastion, Saix scares off Axel and reveals Organization 13's intentions to Sora. He taunts Sora and defeats Maleficent with ease.
The next appearances are my guesses since I haven't played the game since I first got it. Once Kairi and Namine escape, Saïx orders Kairi to see Sora with him, but fails due to Riku's intrusion. After this, Saïx meets Sora and oversees the infiltration and talking a bit about Kairi as well. Maleficent and Pete also show up. After this, Saïx asks Xemnas's permission to kill Sora and mentions something about ending a charade. His final appearance in the game is the boss battle against him and he dies.
If you feel like seeing for yourself, here's a list of the scenes I can remember at the moment:
069- Twilight Town Revisted
081- The Organization's Intent
113- Drawn Together
117- Saix Boss Battle
Kingdom Hearts II Final Mix +
Aside from what already happens in the first game he made his debut, Saïx has few new scenes in Final Mix. He only gets a few words in in one of the many scenes where Organization 13 sit in their meeting room, discussing plans. Another scene, Saïx reports directly to Xemnas in that same white room, reporting what he said to Sora and talking about how Axel behaves as if he has a heart.
Kingdom Hearts 358/2 Days
This game isn't even out yet. However Saïx does show up in Roxas' game. Saïx is seen at first as a playable character. And in one stunning cut scene, you can see Saïx ordering Axel to get rid of the traitor, Marluxia. I didn't expect the Nintendo DS cutscenes to outdo the PS2 cutscenes. Even if the COM FMVs were impressive.
For an evil dude, Saïx is one talkative mofo.
"True we don't have hearts. But we remember what it was like. That's what makes us special."
When Donald points out Nobodies don't have hearts during this first encounter at Twilight Town.
"If I had a heart this is where I'd die of laughter."
After Kairi refuses to go with him to see Sora.
"Don't be reckless. Do you want to end up like Riku?"
After Sora suggests to jump in after Saïx during the first encounter.
"Can you can feel it, the moon's power."
At the start of the battle.
"All shall be lost to you!"
Berserk Stance while close to dying.
"I misjudged you."
After you knocked Saïx out of his Berserk stance with Magma Storm.
While going Berserk. He throws his Claymore at you.
After you knock his weapon from him hands.
"Moon shine down."
Absorbing Moonlight to enter his Berserk stance.
Crashing down on you in Berserk mode.
Saïx roaring while Berserk.
"I want to see you die fighting."
Something he doesn't say in a regular Boss Battle. People have used a cheating device to unlock this phrase from him, while fighting doubles of him. This was taken out from the original game because it's too harsh for the US standards. Many changes have been made to KH2, including Axel's suicide attack, and the color of the Hydra's blood.
Kicking Saïx's Ass
Health Points: 1500
Reaction Commands: Berserk, Eclipse, and Magna Storm
Experience Points: None
Victory Bonuses: 1 Drive Gauge for Sora, 4 HP for Donald, and 1 Item Slot for Goofy
As badass as he was, sadly Saïx was probably the easiest to defeat in the entire game. All you had to do to defeat him, is avoid his Berserk attacks, use the Magma Storm Reaction Command to knock him out of his stance, and pummel him. At times Saïx even stands around like a dork in a trance and actually lets you abuse him. If he hit you at all while Berserk, surprisingly you take very little damage. Even if it does look like such a devastating attack. If Saïx, like his Berserker Nobodies, actually parry your attacks, perhaps Saïx would've been more challenging.