They're Kamen Rider series that has the primary Kamen Rider as a detective and involve them solving case that involves the antagonists. The main characters of each series are very well liked and brings a very cool or should I say hard-boiled feeling to them.

They're very similar in many ways to the point that Kamen Rider Agito was originally supposed to be a sequel of Kamen Rider Kuuga. They both have a dark tone to the series, though Kuuga is much darker.