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.