Do you allow a character to take over a book?

Working on the second in the Wolfe series - a mystery - and I have a character based on a friend of mine. That character is emerging as a larger part of the story than I first intended. In fact, this 'person' may well take focus away from Wolfe. At this point (still on the first draft) I'm going to just let it happen. Then revise as necessary. Interesting what happens in the process, eh?