Basically, this version of Murasame Shizuka is exclusive to this manga series. Muraeda-sensei drew inspiration from many old ZX manga and novels, including a detail from the manga version by Joe Azuchi where Murasame's sister was revived as a cyborg by BADAN. From there, he integrated it directly into Japanese mythology featuring the three powerful sibling deities - Susanoo (JUDO), Amaterasu, and Tsukuyomi - turning Murasame Shizuka into a proper Rider (While Ryo serves as JUDO's vessel, Shizuka acts as Amaterasu's vessel, and for Mikage, well, just himself lol).
And yes, she has an unique Henshin pose.