Anti-Saïx is a character who can be found exclusively as a boss in Kingdom Hearts 358/2 Days. However, it is only found in Mission Mode, and therefore is considered non-canonical. Despite its similar appearance to a Heartless, Anti-Saïx is categorized as a Nobody.
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 its battle right before it summons his weapon. It is a lighter color than Anti-Saïx's skin.
Anti-Saïx does not come into the game's main story, however mimes the original Saïx's role in Mission 91. Anti-Saïx stands on the player's way preventing him from escaping the Castle. After a long fight, Anti-Saïx falls, claiming Kingdom Hearts's strength, while the player accomplishes the mission and returns to the Mission Mode screen.