Rei Kashima is Yuu Kashima's younger sister and a middle school student at an all-girls academy a few stations away from Roman High School. She's a devoted fan of *Let's Fall in Love*—devoted enough that she actually believed Nozaki when he claimed to be the author, making her one of the first people outside his immediate circle to accept that claim.
Being surrounded by girls at school and deeply into shoujo manga has shaped how she sees the world. She tends to read romance into basically any interaction between guys and girls, getting swept up in the drama of it all. It's the kind of thing that comes naturally when you're constantly consuming love stories.
Rei showed up starting in Volume 11, though Nozaki's character profile had hinted at her existence before that. She hasn't gotten a full character breakdown in the manga's bonus pages yet, but she did make it onto the cover of Volume 12.
Content compiled by AnimeList.moe from publicly available sources.
