Start from Scratch and Become a Kamen Rider

In Japan in 2013, a young man, Etsu Shimizu, who was traveling alone in Yamanashi, met a girl named Mari Shinonatsu with a belt-like device. When the two met a humanoid gray monster, Mari, who owned a belt but could not use the belt to transform, forced Shimizu to use it, but successfully transformed into a "Kamen Rider", and it seemed to trigger a certain trait in Shimizu... The story begins with this. Of course - Kamen Rider is definitely not just one person...