In Assassin's Creed Syndicate, the Rooks are the London criminal organization led by Jacob and Evie Frye.
The Frye twins build the Rooks in 1868 as an allied gang capable of challenging the Blighters, a rival organization controlled by Templar interests. The Rooks help the Assassins reclaim districts, protect allies, and undermine the influence of Crawford Starrick across Victorian London.
Jacob generally focuses on expanding the gang and confronting organized crime, while Evie works more directly on Assassin objectives and the search for an Isu artifact. Their different approaches create the game's two-character structure, with missions designed around their contrasting abilities.
The Rooks are fictional, but Syndicate places them among real historical figures and institutions, including Queen Victoria, Charles Dickens, and Scotland Yard. The Blighters are the most likely mistaken answer because they are another major gang in the same city, but they oppose the Fryes.