Kono S wo, Mi yo!: Cupid no Itazura

A 22-year-old man named Rin is troubled by a birthmark on his buttocks and a one-sided affection for an older woman who sees him as a sibling. He has avoided letting any woman see his bare backside since a childhood incident that deepened his embarrassment. Upon discovering that his birthmark might give him supernatural influence over women, he considers whether it could help him in his personal life.