バジリスク~甲賀忍法帖 (Bajirisuku ~Kōga Ninpō Chō) I like the story... the character and also the opening theme. I even bought the novel(in chinese) when I was in HongKong.
Yes, it's not masked rider... it has become a phenomenon in niconico. Girls dancing along with popular song (anime theme, game music, etc) & masked of course. Yeah, Japs girls are cute.