Nobuko Yoshiya was a Japanese novelist active in Taishō and Shōwa period Japan. She was one of modern Japan's most commercially successful and prolific writers, specializing in serialized romance novels and adolescent girls' fiction, as well as a pioneer in Japanese lesbian literature, including the Class S genre. Several of her stories have been made into films.

Class S , or S kankei, abbreviated either as S or Esu (エス), is an early twentieth-century Japanese wasei-eigo term used to refer to romantic friendships between girls. The term is also used to designate a genre of girl's fiction which tells stories about the same, typically focused on senpai and kōhai relationships wherein one girl is senior in age or position to the other. The "S" is an abbreviation that can stand for "sister", "shōjo" , "sex", "schön", and "escape".