Jacob Frye leads the first playable sequence and shares the protagonist role in Assassin's Creed Syndicate. He is an Assassin operating in London during the Industrial Revolution, alongside his twin sister, Evie Frye.
Jacob is impulsive and focused on direct action. He forms and leads the Rooks, a street gang that helps challenge Templar control over London's criminal underworld. Evie is more methodical and specializes in stealth, investigation, and locating a powerful precursor artifact.
Syndicate allows players to switch between Jacob and Evie for most story activities, although some missions are assigned to one twin. Because both are playable protagonists, calling Jacob the only hero would be inaccurate. The question asks who leads the first playable sequence: that sequence introduces Jacob before the game expands its focus to both Frye twins.