Saix has power over the moon element, and he confuses Sora in their first meeting, in KH II, using Roxas's existance. he's the second in command in the Organization, and uses a giant axe-like weapon, a claymore. he's the berseker in the Organization, (he takes his power off the moon to enter on Bersek mode), and one of the final bosses. In KH 358/2 Days, he's the one who gives the missions to Roxas, and Roxas fights him in the end of the game, to go out of the Land That Never Was. He's serious and silent, and he has a "x"-like scar in his forehead, and a awesome blue hair (love blue hair *o* )
The battle with him is not hard, just make sure you grab his claymore when it falls and hit him, and do your best to dodge his energy attacks, staying out of range until you get a chance to hit him. It' shouldn't be hard, if you do it right.
Since he's second in command, he knows of all of Xemnas's plans, and he is the one who ordered Axel to kill Zexion and Vexen, and who ordered him while he was a double agent, spying on Marluxia's conspiration.