I think Lisbeth and Mikael brought out the best in each other. In her interior monologues,Lisbeth reflects that they worked well together, and that not only was being in his company endurable, she enjoyed it very much. She did not have that sort of connection with anyone else,although she grew closer to Miriam Wu in TGWPWF. Their interactions provide the only intentional humor in the first book,especially the scene in which they first meet and when they first have sex.
Many don't realize that Larsson intended there to be 10 books in the millennium series. His life partner has three hundred pages of a fourth novel with Blomkvist and Salander in Nova Scotia Canada. Apparently he outlined many of the other potential novels as well before his death. However the dispute between the Larsson family and Eva Gabrielsson, his partner, over his estate has curtailed the thought of her or another writer publishing these future stories under his name in the near future.