Erika owns most of Millennium. She's Blomkvist's boss, and his long-time lover. She's also married to Greger Beckman, an artist who understands the irresistible attraction between Berger and Blomkvist, and lets them do their thing. Erika is smart, sexy, and – yep – unconventional.
And she's a major thorn in the sides of the other women Blomkvist gets involved with. She doesn't mind them, but they see her as a serious threat. At the end of the novel, when Salander goes looking for Blomkvist to reveal her deepening feeling, she sees him with Erika and aborts her plan, perhaps permanently. Of course, Berger is in the other two Stieg Larsson novels.