Fudanshism - Fudanshi Shugi

Miyano Amata, known as the "Perfect Prince" for his intelligence, looks, and cool personality, is asked to substitute for his sister as an attendant at a doujinshi convention disguised as a loligothic girl. There he meets a girl from his class and they become friends, but he must crossdress in order to be with the girl while learning about the fujoshi fandom.
(Source: MU)