Shoujyo: The Adolescent (2001)

Tomokawa is a tough guy turned bored cop who spends much of his time sating lonely housewives and looking after retarded teen Sukemasa. One day while hanging out at a bar, he is approached by a 15-year-old enjo kosai named Yoko, offering a round of illicit sex in exchange for cash. Though he demurs, their paths cross again and soon a relationship of sorts forms.