Kasuga is the main character of Aku No Hana.
Takao is a reserved middle school student with a love for books, particularly a book of poems from Charles Baudelaire called "Les Fleurs du Mal" (Flowers of Evil). Forgetting his book while at school, Takao returns for it and comes upon the gym clothes of his love interest Nanako Saeki. On impulse, he steals the gym clothes and becomes burdened with guilt and shame for the act when rumors go around school that "a pervert" stole Saeki's clothes. His problems worsen when the school's creepy quiet girl, Sawa Nakamura, confesses to have witnessed the theft and blackmails Takao into a contract with her or she will tell the whole school what he did.
Forced into Nakamura's following, the girl believes Takao to be a kindred spirit in having depraved thoughts and forces him into a series of situations to enact his "perversion" onto Saeki. The events lead Saeki and Takao to start dating, despite Nakamura's interventions pushing the limits of Takao's mental barriers. Eventually though, Takao's feelings start shifting towards Nakamura as he gives into her demented thoughts on life and the two commit depraved acts on their school and the town.