There have been a few of female Riders, but they were short-lived characters because they had a bad habit of...well...dying. For sure there was
Femme, whom I think was exclusively in the KR Ryuki movie (i.e. not in the actual series),
L'Arc (movie-only character from the KR Blade ~Missing Ace~ movie), and
Shuki from KR Hibiki (she died after appearing in only 2 or 3 eps). There were also a couple of female characters in KR 555/Faiz that wore the "Rider Gear" for a couple of fights, but personally I don't really consider them actual Riders because of how it was written in the story (not really something that can be explained in words, it's easier to understand if you see the actual eps in question). Those are the ones I can think of off the top of my head, not sure if I missed any others.
The whole female Rider thing has actually gotten a fair bit of discussion/debate amongst the english-speaking toku fandom, usually along the lines of "they need to make a female rider that doesn't die/get killed".