Which Assassin's Creed game introduced the series' first playable female lead in a mainline dual-protagonist campaign?

Assassin's Creed Syndicate introduced the series' first playable female lead in a mainline dual-protagonist campaign.

The game features twins Evie and Jacob Frye as its two central playable Assassins. Evie is the first woman to serve as one of the principal playable leads in a mainline Assassin's Creed entry, while Jacob is her brother and the other lead. Players can switch between them during much of the story, although some missions are assigned to a specific twin.

Syndicate is set in London during the Industrial Revolution. The Frye twins arrive in 1868 and work to weaken Templar control over the city by building an Assassin gang and recruiting allies.

This distinction is separate from Liberation, which had already featured Aveline as a playable female protagonist in a spin-off. Syndicate is the relevant answer when the question specifies a mainline dual-protagonist campaign.