In Assassin's Creed Syndicate, Evie Frye is the twin who becomes a playable protagonist in the main series.
Evie and her twin brother Jacob are Assassins operating in London during the Industrial Revolution. Players can switch between them for much of the campaign, with Evie emphasizing stealth and Jacob generally favoring direct combat. Their different abilities support contrasting approaches to missions and exploration.
Evie is notable because Syndicate is the first main-series Assassin's Creed game to feature a woman as a playable protagonist. The spin-off Assassin's Creed III: Liberation had already featured Aveline de Grandpré as its lead, but it was released as a separate handheld-focused title rather than a numbered main-series entry.
Kassandra later becomes the canon protagonist of Odyssey, while Aya is a central character in Origins. Those characters are important, but they do not fit Syndicate's specific distinction. Evie's role is therefore best understood in the context of the main series and the game's dual-protagonist design.