how come she isn't in the poster?
is this the only series the have a female rider?
I am sorry if I ask to many questions

Because she's not a central character, I guess. At least not as important as Kit and Len.

Nope. In addition to Ryuki's Femme (who Siren is based on), Hibiki featured a woman who transformed into Kamen Rider Shuki, and before them both was Kamen Rider Amazon's sidekick Tackle.

maybe the next series they do they can make a female rider a central character
or maybe the main character that would be very cool
yes I know we still have to wait for this series to what could happen