Quinn is the most interesting character in the game, period. Also, just because we lost interest in Maya and Josy doesn't mean they are badly written characters. They are just not for us. If you paid any attention or did some scenes with Quinn, you would see she's wearing a mask(which is attractive af imo) and she wore that mask for so long, she nearly became one that mask. At first on 0.3, if you took her offer with Riona, she shows a hint of affection when MC kisses her. With 0.4 if you chose to go to Sage, when you enter her room and try to kiss her, she kicks MC out right away and then sits with a sad face (well, even if you go for the other option she still does that). She actually cares for MC. Sage telling MC Quinn won't stop talking about him should be a sign as well. I'm just not sure if she's scared of someone discovering this side of her, or she just doesn't want to reveal she also has feelings. Her motives are mysterious but she holds significant power and I am guessing she loves to be in control. Of others, of money, of boys, of drug trafficking, of her life, of her restaurant, you name it. You can say many characters are blank but Quinn is the last one you can call blank.
Nothing she did contradicts her actions and knowing a character's background changes nothing. Hell, many people were white knighting the hell out of Maya before they knew her background. We just learned it with 0.4.
Personally what made me lose complete interest in those 2 is the fact that THEY DIDN'T EVEN TELL EACH OTHER WHAT THEY DID WITH MC. They didn't admit they cheated on each other, with the same guy in similar situations (which would be easy to guess that either of them would feel the same way)
TL;DR: Neither Quinn, nor Maya and Josy are blank characters. The latter just fucked up big time.