Kamen Rider 555 (2003)

Very Dark Tokusatsus aimed for a more mature audience.

-Both Kaiju/Enemies in these two kill humans in brutal ways. Both contain graphic violence, and the use of alcohol.
-Mature themes, both are not for children.

-If you like Tokusatsu grim dark, give these two a try. If you still like lighter and softer go watch Sailor Moon or Super Senttai.

555 is the series after Ryuki so if you want to watch the Kamen Rider seasons in order then watch 555 after Ryuki.

Both Kamen Rider 555 and Kamen Rider Blade has dark themes and serious characters. The kamen rider belts are physical and are designed similar to each other. People actually die in the series and they both have similar vibes except for some episodes in blade like episode 30.

While at first glance the two series may appear to not have a common trait, both series have a sole company that was responsible for the existences of Kamen Riders in their own universes. And while the Orphenochs and Humagears/Magears were exact opposites of each other (humans that were once dead and gained monster abilities versus human-like machines with an ability to turn into monsters once corrupted) their dilemmas and reasoning to revolt against Kamen Riders and humans, in general, were quite similar.