I would definitely add: Data.
Sherlock Holmes is most definitely an intellectual. He’s too much of an individual to be called textbook example, but he definitely reeks of it. It’s not only in how smart he is and how well he cracks his cases (which, btw, demands a lot of creative thinking), but in his whole attitude and how England is seemingly his own little sandbox.
He is knowledgable, he plays the violin and he’s a boxer. I dare say that even when he boxes, he boxes “with his brain”, as it were.
If that qualifies P.D. James, then we should mention George Smiley, the master spy in several of John Le Carre’s novels. George Smiley’s academic interest is 17[sup]th[/sup] century German literature. (ETA: I should have noticed that there is a page 2 to this thread, I see that Linty Fresh has already named George Smiley.)
And we should also mention Augustus S. F. X. Van Dusen, the Thinking Machine of Jacques Futrelle’s stories, who “was a Ph.D., an LL.D., an F.R.S., an M.D., and an M.D.S. He was also some other things — just what he himself couldn’t say — through recognition of his ability by various foreign educational and scientific institutions.” (quote from The Problem of Cell 13)
Holmes almost defines the word. An intellectual can solve problems in one discipline by applying lessons learned in an unrelated one. While Holmes wasn’t literature’s first detective, he did distinguish himself by being the first to use medical methods of inquiry to solve crimes.