pervert

When a character does something sexually provoking, such as peeking (and/or taking pictures) with self-gratifying intent, fantasizing about sex, reading pornography, molesting another character, stealing someone's underwear for sexual purposes, etc.

Simply masturbating doesn't count, unless the character is also doing one of the things mentioned above or is masturbating in public.

If the character in question is female, use female pervert.