Camilla Salander is the estranged twin sister of Lisbeth Salander. She's hasn't been seen since their seventeenth birthday, when she and Lisbeth had a confrontation that resulted in Camilla getting a black eye, and Lisbeth a swollen lip.
Apparently, she's very beautiful, and also something of a daddy's girl. Zala speaks of her with something like tenderness. Overall, she seems to be the complete opposite of her sister Lisbeth. Camilla and Lisbeth never had a good relationship, while Camilla thrived socially and academically, Lisbeth was the complete opposite. They began distancing from each other from then on.
In The Girl in the Spider's Web, Camilla is revealed to be a manipulative sociopath who subjected her sister to psychological torture, and sided with her father when he abused her mother we finally find out what has become of Camilla, when she tries to seduce Blomkvist in order to gain access to her sister.