In Assassin’s Creed Syndicate, Jacob Frye leads the playable Assassin missions as one of the Frye siblings.
Jacob and his twin sister, Evie Frye, are the game’s two playable historical protagonists. They travel to London in 1868 and establish a gang called the Rooks while fighting Templar influence during the Industrial Revolution. Jacob is generally associated with direct combat, gang warfare, and taking control of London districts, while Evie specializes more heavily in stealth and searching for an Isu artifact.
The question asks for the sibling who leads the playable missions in the singular sense of the game’s paired heroes: Jacob is the male lead and the character first introduced in the opening sequence. Evie is equally playable and central to the story, so she is a plausible alternative, but she is not the answer requested here.