What or who is a booty-humper?
The sentence is
Well,” Heller began, “at the tattoo place you were a wild booty-humper. You were kissing Billy for a really long time—no, I would say that you were devouring him. All I could see were his feet sticking out of your mouth.

Question

Answer

It is a sexual term.
Stop reading now if you wish :)
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Short version: "frequent sex"
Booty = posterior. humper = "one who has sex with ..."
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As Igor says 
https://www.urbandictionary.com/define.php?term=Humper
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Plus 
https://www.urbandictionary.com/define.php?term=Booty
"Booty" seems an American slang meaning a variety of things, from "your bottom" to "sex".
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Never take the "urban dictionary" too seriously. Some terms listed there are commonplace and frequently used, others are local or used by an extremely small popuation.

Answer

It's prey

Answer

https://www.urbandictionary.com/define.php?term=Humper
sense 2

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!