In Assassin's Creed Rogue, Shay Cormac is the protagonist who begins as an Assassin before joining the Templars.
Shay Patrick Cormac is an Irish-American Assassin operating in the North Atlantic during the eighteenth century. After a mission connected to an earthquake in Lisbon, he becomes disillusioned with the Brotherhood's methods and leaves the order. He is eventually recruited by the Templars and hunts former Assassin allies, making the game's central perspective unusual within the series.
Liam O'Brien is Shay's fellow Assassin and mentor figure, not the protagonist who changes sides. Haytham Kenway is a Templar protagonist in Assassin's Creed III, while Charles Lee is a Templar antagonist in that same game. Those connections can make the names easy to conflate.
Rogue was released in 2014 and bridges stories from Assassin's Creed IV: Black Flag and Assassin's Creed III. Its naval gameplay uses the Morrigan, a heavily armed ship suited to the icy North Atlantic setting.