If you have an extreme character, you need normal characters to contrast them. Sherlock Holmes certainly needed a Dr. Watson. And Pippi Longstocking, who supposedly inspired Lisbeth Salander, needed Tommy and Annika, the normal middle-class neighbors.

My name is Sherlock Holmes. It is my business to know what other people do not know.

You have been in Afghanistan I perceive.

I'm not a psychopath, I'm a high-functioning sociopath. Do your research.

They say that genius is an infinite capacity for taking pains," he remarked with a smile. "It's a very bad definition, but it does apply to detective work.

Well, and there is the end of our little drama," I remarked, after we had sat some time smoking in silence. "I fear that it may be the last investigation in which I shall have the chance of studying your methods. Miss Morstan has done me the honour to accept me as a husband in prospective."
He gave a most dismal groan.