Nancy Callahan is a fictional character in the film "Sin City" (2005), based on the graphic novel series by Frank Miller. She is played by Jessica Alba and is a dancer at the Kadie's Club Pecos, a seedy bar in Basin City, a corrupt metropolis known as Sin City.

Nancy is depicted as a beautiful and innocent woman who is caught in the middle of the city's violent and dangerous criminal underworld. Over the course of the film, she forms a close relationship with the main character, Hartigan (played by Bruce Willis), who is a police officer trying to protect her from the city's criminal elements. Despite her vulnerability, Nancy is also depicted as being resourceful and determined, and she becomes a central figure in the film's overarching story of justice and redemption.

In "Sin City", Nancy's character is heavily stylized, with heavy use of black and white imagery and comic book-inspired visual effects. Jessica Alba's performance is characterized by her striking beauty and vulnerability, and she plays an important role in the film's dark and atmospheric world.