Tristan is in charge of all the day-to-day operations of the Citadel and assisting Paladin Gunny with 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).
Tristan has climbed through the ranks of the Brotherhood of Steel and finally attained the status of paladin, one of the highest ranks. He acts as an operation specialist, planning many of the offensives in defeating the Enclave in the Capital Wasteland.
Blood Brotherhood: Following the destruction of Adams Air Force Base, Tristan asks you 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.
He is the acting field commander of the Brotherhood forces in light of Sarah Lyons being unable to fight after the events at the purifier, and is in charge of all Brotherhood operations involving the player and Liberty Prime, the first mission in the main Broken Steel questline.
"Well, you saw most of the fighting outside. The robot paved the way for us, we just mopped up the opposition while you and Sarah headed inside. After that... Hell if I know. Once we'd locked down the perimeter, and gotten word that the purifier was running, we headed in to look for you. Found you and Sarah both unconscious, and got you back to the Citadel fast as we could."
"With the setbacks the Brotherhood has suffered, we find ourselves in a difficult position. All our available resources need to be positioned for the inevitable counter-attack from the Enclave. At the same time, we believe we may have developed a tool to help our chances of victory. But with everyone needed to help defend against the Enclave, we have no one to send on a recovery mission. So I'm going to have you travel to the Olney Powerworks to secure some tech for us."
"I need you to go there, and quickly. That's the best chance to locate a key piece of experimental equipment, called a Tesla Coil. Information we've gathered suggests it's the best place to look for one, and for what the Scribes are cooking up, it's absolutely essential. If you can find one, get it back here right away. We don't have much time."
PCXbox 360 When you first encounter him in the Rockland car tunnel, he will sometimes be unarmed and in the "fist-fight" stance; however, pickpocketing will reveal that he is carrying a plasma rifle and simply has no ammunition. He will find some ammunition by himself after Liberty Prime kills the first Enclave soldiers, but you can also reverse-pickpocket him some microfusion cells while in the tunnel which will make him fully operational after loading the next screen. [verified]
PCXbox 360 If Tristan suffers a crippled leg during the assault on the Communications station, he will spend the rest of the game crippled never bothering to heal it.[verified]