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 your first encounter, and stays that way throughout your 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 coup d'état with his men against Protector McGraw, feeling that the player doesn't deserve any of the technology within the armory.
If you enter the Outcast Outpost wearing Outcast power armor, found on many of the Outcast reinforcements outside of the base who are killed, he will mistake you for one of them, and will take you to McGraw thinking this. On the way, many of the other Outcasts will assume your unit was one that recently left Lyons and the Brotherhood.
If your player is a female, and you have the Black Widow perk, a unique dialogue option will appear when you enter the base and speak to Sibley. He will lead you to Protecter McGraw without the aggression. He still turns on you 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.
"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."