Erika is editor in chief of Millennium magazine. In Fire she's been offered a job as editor in chief of a high profile newspaper, and she's trying to figure out how to tell Blomkvist she's taking the job. Since he's so busy investigating, they never have that talk. She and Blomkvist have known each other and been lovers for over twenty years, ever since they attended journalism school together. They are also best friends. She's been married to Greger Beckman for fifteen years.
Erika is most important in this novel for her unconventional attitudes toward sex. She's married but has a lover (Blomkvist) with her husband's consent, and she loves to have sex with her husband and other men. She a strong woman, from a wealthy background who doesn't care what people think about her sexuality.
For more on Erika, see our discussion on her in The Girl With the Dragon Tattoo.