Tristan is in charge of all the day-to-day operations of the Citadel and training new members. He follows Elder Lyons's ideals on the new direction the East Coast faction of the Brotherhood of Steel is taking very closely. He is serious and doesn't take any crap, but he's also very wise. Like a few others, he was a part of the original expedition that discovered the Pentagon ruins with Elder Lyons. He has a close (professional) relationship with Sarah Lyons whom he trained since she was quite young. Tristan is usually found wandering the main control room area of the Citadel (the old bay for Liberty Prime).[1]
He is the acting field commander of the Brotherhood forces in light of Sentinel Sarah Lyons being unable to command after the events at the purifier.
Blood Brotherhood: Following the destruction of Adams Air Force Base, Tristan asks the Lone Wanderer to collect super mutant blood samples, so the scribes can research and develop a weapon that can destroy the super mutant population once and for all.
"Well, well. Look who's back amongst the living. Some of us were starting to wonder if you and Sarah were going to pull through."
"[Good to see you, too. I'm here to help.] Basic rule of thumb on these operations: keep your head down and let the robot do most of the heavy lifting. He can take it better than you can."
"[Has the Enclave been putting up much of a fight?] Not like they did at the purifier, no. We got the sense that after you took care of their headquarters, they didn't have anyone to give orders. So they haven't been as coordinated. Some surrendered, some tried to hold out... it's all ended the same. But don't let that fool you. If we can keep the pressure on them, we win. If not, we give them a chance to re-organize... Well, we don't want that to happen, do we?"
Xbox 360 It is possible for Tristan to suffer a crippled leg, which will remain crippled for the duration of the game.
References
↑Fallout 3 Official Game Guide Game of the Year Edition p.89: "Paladin Tristan Tristan is in charge of all the day-to-day operations of the Citadel and training new members. He follows Elder Lyons's ideals on the new direction the East Coast faction of the Brotherhood of Steel is taking very closely. He is serious and doesn't take any crap, but he’s also very wise. Like a few others, he was a part of the original expedition that discovered the Pentagon ruins with Elder Lyons. He has a close (professional) relationship with Sarah Lyons whom he trained since she was quite young. Tristan is usually found wandering the main control room area of the Citadel (the old bay for Liberty Prime)." (Fallout 3 Official Game Guide Game of the Year Edition Wasteland Census)