The Girl with the Dragon Tattoo
Anita is Cecilia's younger sister, who looks and dresses just like her. She is Harriet's closest female friend – before and after Harriet disappears. But for most of the novel, Blomkvist is barely aware of her. He has Cecilia on the brain instead, and thinks Anita and Cecilia are the same person in the photos of Hedeby Island from the day of Harriet's disappearance. The fact that she's in London while Blomkvist is investigating helps keep her out of his radar. Plus, he assumes someone did Harriet harm, and Anita doesn't seem a likely suspect. When he realizes Anita must have helped Harriet escape her brother, he's able to solve the mystery.