Defender Sibley

 is a member of the Outcasts located at the Outcast Outpost at Bailey's Crossroads. Upon beating the Operation: Anchorage quest and entering the chamber containing the Gauss Rifle and other weapons, Sibley will attempt a coup d'etat with his men against Protector McGraw, feeling that you don't deserve anything in the chamber.

Related quests

 * Aiding the Outcasts

Appearances
only appears in the Fallout 3 Operation: Anchorage DLC.