In Assassin's Creed Syndicate, Jacob Frye becomes the playable Assassin in London first.
Jacob and his twin sister Evie are the game's two central protagonists. They leave Crawley and arrive in London in 1868, where Jacob immediately pursues a plan to weaken the Templar-controlled gangs and establish a local Assassin presence.
The opening sequence emphasizes Jacob's direct, aggressive approach. Evie remains playable as well, but her missions generally highlight stealth, investigation, and the search for Pieces of Eden. Players can switch between the twins during much of the campaign, while some missions are assigned specifically to one character.
Evie is therefore the most plausible alternative, but she does not arrive as the first playable twin in London. Henry Green is an Assassin ally and Maxwell Roth is a gang leader, not one of the Frye twins.