She's one of the legendary Four Lords of Pastimers and a student at a nearby all-girls academy. One of Chisato's friends—a teacher there—brought her into the circle, and she first showed up at the Pastimers Club during Episode 4 just to hang out.
Her name doesn't get revealed until chapter 73, which is when fans also learn she's genuinely talented at multiple things: she performs rakugo and plays the koto.
Content compiled by AnimeList.moe from publicly available sources.
