Shay Cormac becomes a Templar after leaving the Assassins in Assassin's Creed Rogue.
Shay Patrick Cormac begins the game as an Assassin operating in the North Atlantic during the Seven Years' War. After a disastrous mission involving an Isu artifact, he rejects the Brotherhood's leadership and joins the Templar Order. The story then presents major events from a Templar perspective rather than treating the Assassins as the unquestioned heroes.
Shay hunts former allies and other Assassins while using a modified version of the Morrigan, his ship. His change of allegiance is the central narrative premise of Rogue and explains why the game connects thematically to both Assassin's Creed III and Black Flag.
Liam O'Brien is Shay's former Assassin colleague. Arno Dorian belongs to Unity, and Charles Lee is a Templar figure in Assassin's Creed III, so neither is Rogue's protagonist.