Kamen Rider Ryuki was remade for American audiences, which included changing every actor, altering the plot, and choosing the most bland white men to replace every charactor. It's honestly so cursed.
Kamen Rider Femme is the first official female kamen rider. Electronic Wave Humanoid Tackle is older, but she's not considered a Kamen Rider because... I dunno. But she's not.
Ryuki broke the mould for Kamen Rider shows by having kamen riders fight each other and be antagonists. This wasn't well liked by older Kamen Rider fans, but it has heavily influenced all Kamen Riders that followed it.
Kamen Rider Knight is the first kamen rider to be based on a mammal (a bat).
There's a Ryuki light novel, but it's absolutely insane. Like, it's grotesque. I won't go into it, but... Asakura eats poop. And that's not even the worst thing about it.