Once the player completes the Anchorage Reclamation simulation and opens the locked door to the armory, Sibley will attempt a mutiny with his forces against Protector McGraw, feeling that the player character doesn't deserve any of the technology within the armory.
Sibley will mistake a player character entering the Outcast outpost wearing Outcast power armor for one of them, and will take them to McGraw thinking this.
If the player character is a female and has the Black Widow perk, a unique dialogue option will appear when the player character enters the base and speak to Sibley. He will lead to Protecter McGraw without the aggression.
Defender Sibley is the weakest human antagonist in Fallout 3, as even at his highest possible level, his stats are even lower than the members of Enclave Squad Sigma from Broken Steel. His S.P.E.C.I.A.L. stats are set like those of a Scribe.
Killing Defender Sibley before entering the simulation will cause the mutiny not to trigger, and the player character is free to enter the armory without any of the Outcasts turning hostile.
"All right, you. Keep your weapons holstered, your hands to yourself, and your mouth shut."
"What's going on is this whole mission is one big rat-fuck. We've been stuck here for way too long with no results. McGraw won't give up, so we've stooped to using chumps like you to try and get something out of this place."
Outcast members Sibley, Olin, McGraw and Morrill, all of whom appear in the Fallout 3 add-on Operation: Anchorage, appear to be named after buildings on the campus of Cornell University, including Sibley Hall, Olin Hall, Olin Library, McGraw Hall, McGraw Tower and Morrill Hall.