Harriet is the mostly unseen star of the chief mystery of The Girl With the Dragon Tattoo. For most of the novel, the other characters think she's long-dead. Blomkvist is skeptical at first, but when he begins realizing that her father Gottfried Vanger was a serial killer, he believes it too.
Only when Blomkvist tracks Harriet to Australia is the tale unraveled. Like Henrik, Harriet confesses her dark family history to him. Here it is: Gottfried Vanger rapes Harriet repeatedly from the time she is fourteen to the time she is fifteen. The biggest surprise for Blomkvist is that Harriet kills Gottfried when she's fifteen, in 1965, a year before she disappears. Her brother Martin, who has also been raping her, witnesses the killing and uses this information to make her his sex slave. While Martin is away at school, leaving her in relative peace, Harriet sets out to identify the women her father killed. (Harriet doesn't talk about this part – we're reading between the lines.)
She sees Martin back in town on Children's Day, and decides to run away. She hides in Anita's car and Anita drives her away from Hedeby Island. Anita gives Harriet money, and her own passport. Harriet falls in love with and marries Australian sheep farm heir Spencer Cochran. As such, she gains a new identity and a new life with her husband and their three children in Australia. So, for the past thirty-some-odd-years, Harriet's been farming sheep and has a prospering company. She's an ace businesswoman with a happy life, except for the burden of her secrets.
At the end of the novel Harriet is back in Sweden, and is to become CEO of the Vanger enterprises. She's become friends with Erika Berger and is an interesting player in the next two novels.
We don't really get to know Harriet because she's missing for most of the book. But, in addition to being at the center of the mystery, and presenting a heartbreaking story of abuse, she's used to present a moral and ethical dilemma.
Blomkvist totally understand why Harriet fled, and why she didn't tell anyone what Martin did to her. She was ashamed, frightened, and traumatized. She was running for her life. Blomkvist is certain that Harriet had no idea Martin was a killer.
But Salander has a different take on the matter. After hearing Harriet's story from Blomkvist, she calls Harriet a "Bitch" (27.12), and says, "Harriet Fucking Vanger. If she had done something in 1966, Martin Vanger couldn't have kept killing and raping for thirty-seven years" (27.13).
Salander thinks that most rapists don't stop being rapists. She thinks Harriet should have known this and done something to make sure Martin wouldn't hurt others. But most people simply aren't avengers like Salander. Harriet keeps quiet out of a complex blend of fear, shame, and self-preservation. She's doesn't want revenge on Martin, nor does she see it as her place to protect others from him. So, what do you think? Do victims of sexual assault have a responsibility to make sure their attackers never attack again?