Cecilia, like Harriet, is Henrik's grandniece. She's 57 now and lives alone on Hedeby Island. Her father is Harald Vanger, Henrik's only living brother, the Nazi who "ruined [her] life when [she] was just a child" (14.39). He's called her a "whore" (17.21) since she was 21 when he found out she lost her virginity to a man with Jewish ancestry. The man Cecilia marries beats her brutally. Although she never sees him, she's still married to the man. For some reason, Vangers don't get divorces. Cecilia doesn't speak her to her father, or to her husband.
She takes a fancy to Blomkvist and promptly seduces him, then falls in love with him. But she isn't down with sharing him with Berger, and ends up liking him too much to be a part-time lover. When he gets too caught up in investigating Harriet, he begins to suspect Cecilia of knowing more than she says. As far as we can tell, she really didn't have a clue about what was going on with Harriet. Blomkvist tries to apologize to her when he visits Hedeby at the end of the novel. Since she isn't home, he leaves her a note. We'll see if she forgives him in the other books!