He was voiced by Julian Casey.
Liam was born in Plymouth, Massachusetts in 1726 to a family of Irish immigrants. The O'Briens struggled on their farm and later moved to New York. Liam joined his father in working for a shipping merchant loading England-bound ships. It was in this business where he met Shay Cormac. Immediately the two became best friends, with Liam often looking after Shay, due to his rough living conditions.
After a few years, Liam's employees were growing in wealth, while their employees were unpaid and mistreated by the authorities. These tensions grew to a breaking point, and Liam recruited several co-workers in a plot to rob their employer and distribute the money among each other. They got caught in the middle of the heist and Liam killed a man in self-defense. To protect Liam, his father took the blame and was executed.
Joining the Assassin Brotherhood
After his father's execution, Liam moved from one job to another and committed theft to help his aging mother. Liam began drifting away from his friends, including Shay. In 1738, he met master assassin Achilles Davenport who tasked him with small errands. Over time, Liam looked to Achilles as a mentor and became influenced by the Assassin order. Achilles eventually inducted Liam as a member, thus making Liam his first student. Liam helped Achilles recruit other members of the brotherhood such as Hope Jensen, Kesegowaase, and Louis-Joseph Gaultier, Chevalier de la Vérendrye.
22 years later in 1748, Shay lost his father and reconnected with Liam, who helped him get back on his feet and recruited him to the Brotherhood. Because of his bond with Shay, Liam was tasked by Achilles to look after Shay, just like he did when they were children.
Assassin Expedition and Liam's death
12 years later in 1760, the assassins found the Voynich Manuscript and Precursor Box, which they thought might lead to a piece of Eden. When Shay was sent to Lisbon to find the artifact, ithe accidentally caused an earthquake that destroyed the city, causing him to betray the Assassins and join their enemys, the Templars. When Liam and Achilles traveled to the Arctic to find a simialr artifact, they were confronted by Shay and Haytham Kenway. Liam attempted to shoot Shay, but was stopped by Achilles, and knocked over the artifact. This caused the area to destabilize like it had in Lisbon and the temple began to collapse around them.
Liam took the manuscript and fled the Temple, but was followed by Shay. Liam revealed to Shay that Chevalier had taken the Precursor Box and hidden it where the Templars would never find it. Shay cornered Liam on a cliff, and the two fought. A tremor from the earthquake caused the cliff side to collapse and both men fell. Shay suffered minor wounds, but Liam's were fatal. He questioned why Shay joined the Templars to which Shay replied "to save the World." Liam sarcastically hoped that it would be a good one before dying. In respect for his old friend, Shay pulled Liam's hood over him.
Personality and Skills
Liam is a patient and respectful person who does not hesitate to intervene between Shay and Chevalier fighting. He is seen as a more peaceful and calm character, but would not hesitate to sacrifice his friendship for the sake of fulfilling the duties of the Brotherhood. He also believed in the Creed and admired fellow Assassin mentor, Adewale. He is a very skilled assassin and a proficient marksman and swordsman. He was also capable of defeating a number of colonial soldiers, and had some knowledge of sailing, acting as the first quartermaster of Shay's ship, the Morrigan. Liam is also a skilled freeruner, being able to perform a leap of faith. He is a skilled man in the field of weapons, equipped with two hidden blades, a sword, and, his preference, dual pistols.
- He is obviously not meant to be confused with the real life voice actor of the same name.