Sai'X real name is Isa. Before he became a nobody, Xehanort gave Isa a part of his heart. Under Xemnas control, he had much power and was extremely mysterious. Sai'X and Axel never got along very well, as he was all for order, and Axel was wild and jumpy all day, any day.
Organization XIII's Luna Divider
This did not please Sai'X or Xemnas at all, as he had no emptions to display. Therefore, he kept the two Organization XIII members away from eachother as much as possible. Sai'X kidnapped Kairi in Kingdom Hearts II, where she was kept prisoner for a majority of Sora's adventure after reawakening. While she was there, sai'X and her talked about a lot of things as Sai'X rarely had to go out into battle on his own. She asked him about lots of things, but mostly about what "Kingdom Hearts" really was. He never told her, but only gave her the really bad news about it:That Sora was helping them get one step closer with each move he made. Every time Sora killed a heartless with the Keyblade he sent their energy up to The World that Never Was, a place that Organization had been using as their headquarters since they were brought into action and realized who and what they were.
Sai'X, after Sora killed many heartless with the help of Cloud and Tifa and Squall in Hollow Bastion, and after he'd found out far too much about Organization XIII and was coming here to fight them. However, after revealing what he had told Kairi to him, Sora lost motivation to fight them anymore, as each move he made sent them closer to their goal.
In the world that never was saix was encountered
in the "Addled Impasse" chamber of the castle where
His final battle took place by absorbing the power from the kingdom hearts moon his recieved beserker power and used savage attacks on Sora, Donald, and
Goofy inspite of that his was defeated. In the end all he wanted was a heart.