An 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.

Yoko, it turns out, is Sukemasa's sister; and both are the children of his old flame Yukie, a grasping, self-centered woman. Yoko's grandfather is responsible for the massive tattoo sprawling across Tomokawa's back. Tomokawa soon takes both teens under his wing, protecting them from their heartless mother, and their lecherous stepfather. Soon, Yoko gets a similarly massive tattoo illustrating her bond with her policeman savior.