A rather morose and suspicious man, Sibley is concerned only with furthering Outcast technological findings. He seeks to share this wealth only with those in the service of the Brotherhood Outcasts. Sibley instantly views anyone entering the Outcasts' domain as a threat, and he doesn't trust McGraw's motives. He thinks the Protector is weak-willed and gives in too easily to those who might steal Outcast bounty.
Sibley comes off as harsh in the first encounter, and stays that way throughout the stay with the Outcasts. He is the one in charge of the mutiny at the end of the simulation.
Once the player completes the Anchorage Reclamation simulation and opens the locked door to the armory, Sibley will attempt a mutiny with his men against Protector McGraw, feeling that the player character doesn't deserve any of the technology within the armory.
This whole mutiny-affair can be prevented by disposing of Sibley before one enters the simulation to begin with. He can be slowly pushed into the locked room with a Gary corpse, then one may find a power fist upstairs and power fist him to death without causing the Brotherhood to become permanently hostile. They will immediately turn red upon his death, but if one waits long enough without them spotting the player character, they turn green and stay friendly.
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. On the way, many of the other Outcasts will assume the unit was one that recently left Lyons and the Brotherhood.
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. He still turns on the Lone Wanderer at the end, though.
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. His low stats usually result in him being killed by Olin, McGraw, and the player very quickly after he triggers the mutiny.
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.
"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."