The title character starts out in the book seeming like she might be a supporting character but it soon becomes apparent that this socially-disfunctional, computer research genius with a questionable sense of morals is a main focus. Lisbeth Salander is a hacker who is brilliant at her job, but her employers aren't aware of just how deep her skills really allow her to look at a person's life or computer usage.
Lisbeth had a very hard life growing up and is a ward of the state. Many think she is mentally-deficient, not realizing she possess genius-level skills and a very personal sense of morality, including her beliefs around revenge. She becomes involved in a journalist's investigation into a missing person's case connected to a wealthy family. Her own drama plays out alongside the main story, with her involvement becoming more and more crucial as the reporter gets closer and closer to finding the truth.
In addition to a personal sense of conduct, Lisbeth follows her own style appearance-wise. She is described in the book as having a variety of elaborate tattoos and a variety of facial and body piercings. Audiences won't know until the film opens if her on-screen portrayal is a colorful as it was in the novel.