It is extremely rare to find a male detective in a mystery series who has a complex, multi-faceted personality which grows and transforms over the course of the series. Male sleuths tend to be assigned a set of quirky characteristics in the very first novel of the series. Then, references to these quirks and tics are made in the subsequent novels to remind us of what this character is supposed to be like. Even Ruth Rendell, whose greatest talent (among many) resides in creating complex, fascinating characters, fails to do so with her Inspector Wexford. In Rendell's Inspector Wexford series, we see this character over the course of fifty years and can safely say that he experiences absolutely no changes in terms of his personality. For this reason, I have always found this series to be quite boring.
Or take, for example, Elizabeth George's Lynley and Havers series. Lynley, the male sleuth, is always the same. If you have read a single one of these novels, you know all there is to know about this character's personality. Female sleuths, however, fare a lot better. They are given personalities that are complex, profound, growing, changing with every new installment. Barbara Havers, the female protagonist of George's series, differs from her male counterpart Inspector Lynley in that her experiences in these novels help her grow. The Havers of A Great Deliverance (the first novel in the series) is not nearly the same person we encounter in This Body of Death, which is the most recent installment.
The same could be said about many other female detectives. Laura Lippman's Tess Monaghan, Lisa Gardner's D.D. Warren, Tess Gerritsen's Jane Rizzoli and Maura Isles are complex, interesting characters whose personalities undergo profound transformations in the course of the series.
I believe that the reason why male sleuths are frequently so flat, cartoonish and boring lies in the tradition of the early classics of the mystery genre. Sherlock Holmes, Hercule Poirot and Father Brown are the models on which the entire cast of later male sleuths was built. These three classic sleuths are quirky and original. They are, however, always the same. Poirot's personality is exactly the same in The Mysterious Affair at Styles as he is in Curtain. As for Holmes and Father Brown, I have read all stories featuring these characters (usually more than once) but for the life of me couldn't figure out the chronological succession of these stories.
(To be continued. . .)