Mimmi is Salander's friend and one of her sexual partners. She's a sociology student, who works at Domino Fashion, an erotic boutique, and writes articles of interest to GLBT readers from time to time. She's also a performance artist.
Like Blomkvist, Salander has an unconventional sex life. In Tattoo, we learn that she prefers men to women, but doesn't really care about labels identifying a person's sexual orientation – so long as the sex is consensual. Her relationship with Mimmi is basically uncomplicated. They value and enjoy each other's company and enjoy sex together, but aren't trying to have a close and serious relationship.
Until Mimmi is taken in by the police for questioning, Mimmi doesn't know anything about Salander – not that she's under guardianship, not that she was a researcher for Milton Security, nor any other personal information.
Mimmi unwittingly gets caught in the crossfire when Zala and his men begin hunting Salander. Luckily, with the help of Paolo Roberto she escapes Niedermann and his chainsaw. She's harassed by the police and slandered by the media when her relationship with Salander is discovered. Mimmi seems very cheery and well-adjusted, a positive person who brings some light into Salander's life.