Hadaly Lilith functions as the artificial assistant to former U.S. President Ulysses S. Grant. She first emerges during a Khyber Pass ambush, saving John Watson and his companions from enhanced corpses. Her origins are explicitly non-human; she is an artificial lifeform designed with unique sound wave manipulation capabilities allowing temporary suppression or control of reanimated bodies, proving strategically vital in confrontations.

A core motivation drives her: the pursuit of a genuine human soul, stemming from her artificial existence. This fuels her collaboration with Watson, notably aboard the USS Richmond and during their San Francisco escape when a broadcast signal triggers corpse hostility. Her interactions reveal a complex relationship with humanity and identity, often questioning her purpose beyond programmed functions.

At the Tower of London climax, she confronts antagonist M and uses her sound-based powers to subdue The One, Frankenstein's original creation, sustaining a gunshot wound from M during the fight. In The One's subsequent ritual to create a bride, she unwillingly becomes the intended vessel for this new consciousness before the plan is thwarted.

Following these events, Watson observes signs suggesting her development of an independent soul. A post-credits scene, set four years later, depicts her using the alias Irene Adler. Accompanied by Frederick Burnaby, she observes Watson alongside a companion believed to be Sherlock Holmes, indicating her continued existence and a potential shift in identity or role after the central conflict.