- Shay: "Connor may inherit the house, but you'll be Mentor."
- Liam: "We'll see."
- Shay: "Whatever you say, Prince Regent of the Assassins."
- Liam: "You call me that again and I'll throw you in the ocean!"
- —Liam O'Brien and Shay Cormac on the former's future.[src]
Liam O'Brien (1726 – 1760) was a member of the Colonial Brotherhood of Assassins operating in the British Colonies during the Seven Years' War. A childhood friend of Shay Cormac, Liam was responsible for bringing Shay into the Brotherhood. In spite of Liam being a few years his elder, Shay formed a brotherly bond with him, even though Liam was wiser and more experienced.
Liam was born in Plymouth, Massachusetts. A family of Irish immigrants, the O'Briens struggled to survive on their farm for years, until they moved to New York, where Liam's father began to work for a shipping merchant. Eventually, Liam joined his father at work, loading ships bound for England. It was at work where he first met a young Shay Cormac, and the two became fast friends. Liam looked after the younger Shay, who lived in a rough neighborhood and had a knack for getting himself into trouble.
Over the next years, Liam became aware of his employers' growing wealth, even as his fellow workers remained underpaid and mistreated by their superiors. Eventually, the tensions rose to a breaking point, and Liam recruited several of his co-workers in a plot to rob their employer and distribute the money among themselves. However, in the middle of the heist, the group was caught, and Liam was forced to kill a man in self-defense. Wishing to protect his son, Liam's father took the blame for the murder and was executed by the authorities.
Building the BrotherhoodEdit
In the coming years, Liam drifted from one job to the next and continued to rob the city's richer citizens, sending the money to his aging mother. He also distanced himself from his fellow friends, including Shay. In 1738, he met Achilles Davenport, who gave him small tasks and errands to perform. Over time, Achilles's influence over Liam grew, and Liam looked to the older man as a teacher. Eventually, in 1743, Achilles inducted Liam into the Assassin Order as his first student, who in turn helped recruit the other members of the Colonial Brotherhood, such as Chevalier de la Vérendrye, Kesegowaase, and Hope Jensen.
In 1748, Liam reconnected with Shay, who had just lost his father, and managed to get Shay back on his feet. The two became more inseparable than ever, and Shay was soon included in the list of inductees. At that point, Achilles tasked Liam with keeping an eye over Shay, just as he had years ago.
In 1760, Liam and Achilles undertook a journey to a First Civilization Temple located in the Arctic, following the coordinates revealed by the Voynich manuscript and the Precursor box. When they arrived, the pair discovered the Temple did not house a Piece of Eden, but one of the artifacts capable of causing severe seismic activity, if it was disturbed. Pursued by Haytham Kenway and Shay Cormac, Liam tried to vengefully shoot Shay but was stopped by Achilles. Liam accidentally fell onto the artifact, causing the area to destabilize and begin to collapse around them.
Fleeing from the Temple with the manuscript, he was pursued by Shay. Liam revealed that Chevalier had hidden the Precursor box where neither Shay nor the Templars could find it. Liam also had an angry exchange between Shay over Hope's death, which Shay mournfully pleaded that he had no choice.
Eventually, Liam was cornered at a cliff side and dueled with Shay. However, the cliff side began to collapse from the earthquake, causing both men to fall. Shay used Liam to break his fall,and the latter was fatally wounded. Questioning Shay's motivation for joining the Templars, he sarcastically hoped that the world Shay was attempting to save from the Assassins would be a good one. Shay then pulled Liam's Assassin hood over him, as a sign of respect and mourning for his former friend.
Personality and characteristicsEdit
Liam appeared to be a patient, pensive and respectful man. He did not hesitate to intervene between the two of his fellow Assassins fighting. His character was mostly peaceful and calm, but he would not hesitate to forego friendship for the sake of fulfilling his duty to the Brotherhood. Liam also unquestioningly believed in the Creed, and felt a sense of great admiration for Adéwalé and his fellow Colonial Assassins, seeing them as people from whom it was necessary and useful to learn. Liam was, considered by Shay, also a "wise" man.
Equipment and skillsEdit
Liam was a highly skilled Assassin, having been Achilles' first student and possessing over a decade's experience in the Order. A proficient marksman and swordsman, he was capable of defeating a number of British soldiers in quick succession. Accompanying this, Liam possessed a knowledge of sailing, acting as quartermaster of the Morrigan.
In terms of physicality, Liam was a skilled freerunner, capable of performing the Leap of Faith. His arsenal consisted of Hidden Blades, a sword and dual pistols, with the last of these appearing to be his preferred weapon. He was not only very confident, but also proud of his marksmanship; when Shay accused him of having been the one to shoot him in the back, Liam denied this, remarking that it was Louis-Joseph, and boasted that he never misses a shot to reinforce his claim.
- Liam is the Irish short form of William, which is derived from the elements wil "will, desire" and helm "helmet, protection". O'Brien is an Irish surname that means "descendant of Brian".
- In the memory "Scars", Liam is described as a "huge man with an axe", despite the fact that he is never seen with such a weapon throughout the game. However, his concept art does depict him with an axe.
- When playing events prior to Shay's defection from the Assassins, a glitch occurs where if the crew spots a prisoner-of-war ship, Christopher Gist can be heard reporting this news rather than Liam.
- A trailer of Rogue shows Shay killing Liam during their fight contrary to the events of the actual game where he dies after falling off a cliff with Shay. 
- In the memory, "Non Nobis Domine", his body can neither be picked up nor looted by Shay after his death.