You answered my comment twice. Also no, Quinn is not the antagonist, not even close to be an antagonist, I don't even know how you got that she is an antagonist.
Tybalt is an antagonist, Dawe and that Hulk (I forgot the name) are antagonists, Buddy and Vinny are antagonists, Stephen is an antagonist and so on.
If I had to use a proper label for Quinn apart from a LI or MG it would be tritagonist. She can be a deuteragonist with a looooooot of strech, but that label fits Derek (and the DIKs to some extent) better.
Which is the reason I said debatable for her being a main girl since she has one of the most well developed paths in the game.
Stories involve well developed antagonists. 2nd, 3rd character...that just describes focus, not what they're doing in a story. I can see why you think that, because she and others have so much content.
Anta or anti is any character that opposes someone, like the protagonist. Deuteragonists for example are mostly antagonists. For example, in a romance story, the male lead is the protagonist, and the female is the antagonist. In depends from what frame.
Tybalt is no different than Quinn for romance stories, or love triangles, even if Jill (the female lead) isn't reciprocating, yet they're still interacting. Which is why he's seen as a comical character, not to be taken seriously. There's plenty of dialogue with the MC and Quinn about romance, and how she doesn't publicly want it, doesn't want schooling, a future, feelings, broccoli, etc. This plays into her bad girl persona.
Quinn is unique in that she plays both roles depending on your path, and the main plot. She tries to make deals with the MC, calls him names, teases him, seems protective of him, seems to have positive feelings in one scene, negative in another, but not the next, definitely can be a love interest, has deals with Stephen and Tommy, is opposed to Vinny and Buddy, and is manipulating the HOTs (i.e. Riona). Riona's backstory is largely a result of Quinn, and is now AWOL, despite the MC being on a Riona path. She caused Mona to leave, and Riona hates her.
She's apparently having a redemption arc for all this, which means she has: 1) done something opposing someone and got caught, or at least aware, 2) feels remorse and wants to change herself.
The problem is the execution of all these ideas and her plot. Why does she need so much money? Now that she's lost it, is she going to get it back? Then all the threads that she brought up that aren't dealt with (e.g. drugs, prostitution ring/restaurant/scholarship, Burke.)