Kamen Rider Agito (2001)

Both Kamen Rider series feature an amnesiac Rider who is 'adopted' by the family of the heroine (which has powers). There are also four main Riders.

Both Kamen Rider Agito and Ryuki share similarities in the dark tone and the mystery that surround the plot of the series.

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.