Knights of the World, But I'm a Weirdo at the Start

Having lived in a parallel world for eighteen years, Lu Zheng always believed he'd traveled to an ordinary world. Until one day, on his way home from school, he encountered a man-eating monster and discovered that this world was home to many familiar creatures. Gurongi, Orphino, Zerg... The only reason they'd been able to live peacefully before was because of the Kamen Riders silently protecting them from the shadows. Lu Zheng: I understand, but why did I start out as a monster?