She was a slut, sure.
Thumbs Up, Thumbs Down and a Wagging Finger of Shame

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Mercy is a messed up girl, living among the working poor, with no hope. All she had going for her was pretty looks (which the film shows are fading fast), parties and numerous hook ups.
She follows the Warriors because they are different, particularly Swan. By the time the subway scene comes up, she has pinned all her hopes for change on him. All she has to offer him is sex but he wants emotional connection. Something that Mercy can't offer him at that moment.
I find the subway scene where the faded Mercy comes face up with a slumming rich prom girl extremely sad. Swan shows her empathy and it changes her. It was what she needed to start her change.

@bizzniss : you know you're insulting people because you're not confident in your argumentation right ? Can't believe Mobocracy replied to you AND didn't return your gratuitous insult. Lucky you.
If your counter argument is that "there [is so] many exerpts where they acknowledge it, I was just poiting ONE out", why don't you point the other ones instead of insulting people ? Answer : cause there are none.
Just watched the movie and noticed absolutely no passage at all that confirm Marcy is a prostitute. Your statement is a personal interpretation and even an assumption wouldn't confirm it but rather that Marcy is nymphomaniac. In the "best case scenario", an extended interpretation would be that Marcy would use sex to gain power, just like Mobocracy suggested.

Mercy is a prostitute who hangs around with The Orphans but decides to tag along with The Warriors after looking for some real action. She takes a shine to Swan and walks with him on their way to Coney Island. She talks about her life as they walk through a subway tunnel where they share a kiss. Swan instantly regrets this and ditches her. They meet up again right before the fight with The Punks where they make up. On their arrival back to Coney Island she takes off with Swan and the other Warriors.
http://warriorsmovie.co.uk/cast

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All you're doing is reading between the lines. I never remember a single line that definitively says she does sex work for money.
You second bit of "proof" above doesn't say "what she does" only that she calls it "a living".
Prostitution isn't unreasonable as an assumption, but it's only an assumption. With her character and where she's from, there's a big grey area between prostitution involving sex in exchange for money and promiscuity used as a means of obtaining influence with the gang leadership or for drinks or favors.

LIKE I SAID IN MY ORIGINAL STATEMENTLISTEN CLEARLY:
"There were many excerpts where they acknowledge it"
, I was just pointing ONE out.
There is another part where Swan says, "I don't like what you do". She say's, "it's a living."
If you can't tell from that, then you are obviously very stupid, unworthy of my time and this conversation is over.

That really could be interpreted many ways either as she just gets drunk and has sex with whoever wants it, or she's an actual prostitute. Nothing in that statement explicitly says she has sex for money.

She's a prostitute. If you Listened to the movie, she clearly states it. It didn't take a rocket scientist to figure that out.
Here's one of the many excerpts where they acknowledge it:
Mercy: Friday nights are good. Saturday nights are better.
Swan: I bet you can't even remember who you get on Friday and Saturday night you probably don't remember what they look like
Mercy: Sometimes I can and sometimes I can't who gives a damn?
Also, She wanted Swan AFTER they shared a long passionate kiss, She's been hot for him all night, that's one of the reasons she followed him, the kiss just set it off. Plus it was risky. If the world were about the end, most people would have sex.

I saw this movie in my teens and after seeing how slutty and horny Mercy was it affected me. I thought man I hope I find a really dirty, slutty girl like her as a girlfriend, (p.s. - mission accomplished).

She was probably just a poor girl living in a poor neighborhood with no hope and no future. Her sexuality was probably one of the few assets she could use to gain whatever advantage she could, socially (to gain favor with whoever had power), economically (as in outright prostitution or in-kind for some other economic gain like rent), or emotionally as an outlet for boredom and the anxiety of a future no different than the women around her married to some loser, poor, stuck with kids and miserable.
Swan just likely was like hitting the trifecta for her someone from outside the neighborhood who seemed to have power and influence. All she has to offer him is sex.

You hit the nail right on the head.

Liking
sex is one thing. Literally
begging
for it, in the worst possible time and place, from a guy who's done nothing but insult her, threaten her and treat her with contempt - that might indicate that the girl's got a problem.

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There's a book? OMG. Gotta find it!
Yeah the little boys in IT banged the girl when they were 12.

in the book they all gangbang her out in the streets right after murdering some random guy. then they just leave her on the ground never to be seen again.

Wrong with her?? Well..maybe she just likes sex?

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Begging Swan to have sex in the subway made me think Mercy had a problem. It wasn't the time or the place for it.

WOOPS had a brain fart there, she's his ex-girlfriend! lol what was I thinking.

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Bad upbringing, probably living a party lifestyle all the time, idiot for a brotherlol but yeah, I don't know about "Nympho" because that suggests she has a disorder. She was just promiscuous to me. But I liked her, she was a nice girl in the end I thought. Just needed to find the right guy, and I guess Swan was that guy.