Mimmi is one of Salander's lovers. We learn that Salander "prefer[s] guys" (18.2), but has a "problem […] finding a guy who was not a jerk, and who was good in bed" (18.2). Mimmi, she thinks, "[is] a sweet compromise […]" (18.2). We don't actually meet Mimmi in this book. All we know is that she leaves Salander's apartment just before Blomkvist shows up for the first time. We'll get to know her better in the next two books of the trilogy.