The Anti-Saïx is a dark version of Saïx that is found in Kingdom Hearts 358/2 Days. It appears as a boss in the Mission Mode version of Mission 91: Escape the castle, replacing Saïx as a boss at The World That Never Was in Nothing's Call. As Anti-Saïx only exists in Mission Mode, it does not exist in Kingdom Hearts canon.
The Anti-Saïx is, as its name suggests, a doppelganger of Saïx. Its coat is completely black, including the zipper and pull strings. Its long hair and skin are both very dark-colored, and it carries a dark copy of the Lunatic claymore. Its eyes are glowing yellow circles, traits possessed by many Heartless. It is unclear if this being possesses Saïx's scar. Something resembles the scar, though, at the beginning of his battle right before he summons his weapon. It is a lighter color than Anti-Saïx's skin.
The Anti-Saïx's name reflects the fact that it is an antagonistic, dark copy of Saïx. The prefix "anti-" means "against".
Anti-Saïx uses the same tactics as the true Saïx while in battle, though he drops less frequently from his Berserk state, making him much more difficult to defeat. It will immediately power up and go berserk at the start. The boss will continue to hammer the player with its dark version of the Lunatic, so it is important to heal regularly and also make use of the Limit Break. Quickly attack the moment Anti-Saïx has settled down, or you will be hit with an attack.