Anti-Saïx is a dark version of Saïx that appears in Kingdom Hearts 358/2 Days. It serves as a boss in the Mission Mode version of Mission 91: Escape the castle, replacing the true Saïx in The World That Never Was at Nothing's Call. As Anti-Saïx only exists in Mission Mode, it does not exist in Kingdom Hearts canon.
Anti-Saïx is, as its name suggests, a doppelgänger of Saïx. Its entire body is black, and it carries a dark copy of the Lunatic Claymore. It has circular eyes that glow yellow, a trait common amongst Heartless. It is unclear if this being possesses Saïx's scar.
Anti-Saïx's name reflects that it is dark copy of Saïx. The prefix "anti-" means "against."
Anti-Saïx uses the same tactics as the true Saïx while in battle, though it drops less frequently from its Berserk state, making it much more difficult to defeat. It will immediately power up and go berserk at the start of battle. The boss will continue to hammer Roxas 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 it will counter the moment an opportunity presents itself.