From Wikipedia:
Hanabuki High School prohibits its pupils from wearing makeup. Among the school girls there is a rumour going around about a legendary make-up artist, Ageha, who secretly gives makeovers to girls after school. Any girl wanting Ageha to give them a makeover needs only to write their name and class on Ageha's card (showing Ageha's trademark, a swallowtail butterfly—agehachō in Japanese) and put it up on the school's notice board, and Ageha will later show up in their classroom. However, no one knows that Ageha's true identity is the quiet, bespectacled girl, Rin Fujiwara, except for the play boy, Riku Kisaragi. Amused by the idea, Kisaragi cross-dresses as Ageha's assistant to tag along with her as she works. A running gag in the story is that Ageha's clients think that Kisaragi is Ageha upon first meeting.