Which Assassin's Creed game first introduced the series' modern-day Abstergo employee Layla Hassan?

Assassin's Creed Origins first introduced the modern-day Abstergo employee Layla Hassan.

Layla appears in the modern-day sections of Origins, where she is an independent researcher who uses a modified Animus to explore the memories of Bayek and Aya. Her work brings her into conflict with Abstergo and leads to contact with the modern Assassin organization.

She returns as a major modern-day character in Odyssey, Valhalla, and related downloadable content. This makes her the central modern protagonist of the post-Desmond era, even though the historical playable character changes from game to game.

A frequent source of confusion is that Origins' historical hero is Bayek, while Layla is the modern-day viewpoint character. Kassandra and Alexios belong to Odyssey, and the modern framing of earlier games is dominated by Desmond Miles.