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, 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.
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.
It is important to heal regularly and also make use of the Limit Break when he is out of his Berserk state. The player should quickly attack the moment Anti-Saïx has settled down, or they will be hit with an attack.