Pure ★ Hustle
The apartment complex our protagonist lives in has had a plumbing accident and his toilet is the only one with the running water. Because of that, both Rin, his childhood friend, and Saeki, his neighbor, get a spare key to his apartments. This eventually escalates to both girls having a competition about who can please him more. Later that day, he meets his landlady drunk on the street and helps her back to her apartment. Turns out she’s less of a frail girl and more of a predator herself...
Bewitched by emotion and the curse, hungering for ecstasy... Suspenseful school fantasy's bold conclusion!