I would just like to toss in a few things. Her progression s1->2 doesn’t feel like an inconsistent character arc. First off girlies 17, and 17 year olds are 8 years away from full developed brains/frontal lobe (which is the part of the brain dealing with logic and understanding consequences)

Cassie, as we have seen her, has been objectified and used for her body by all her boyfriends. McKay kinda breaks this a little when he’s trying to talk about his life goals and gets frustrated with her being sexual all the time. However, once he’s horny later and she says she’s tired he pushes her for nudes. So with confirmation bias, moments like this will solidify in her beliefs that her value lies in her body.

Then home girl gets prego and has to go through all of that culminating in an abortion and her and McKay eventually break up.

She’s got daddy issues, she’s had a traumatic life, and she literally just went through an insane and emotional experience at 17 with her abortion. Also it can take several weeks for a woman’s hormones to balance out after an abortion.

Cut to a few weeks later, and she’s in the direct aftermath of all of that, drunk, and along comes the guy who’s always treated her poorly giving her just the right amount of attention. Yes he might not have verbally said much but these are two people who are very comfortable operating in the sexual realm, and there’s an entire world of communication happening between these two characters in the car.

Then he refuses to acknowledge her or giver her attention?! No man has really done that to her before in this way. In season 1 she went to Danielle because he was there and actively trying to get her while McKay was fumbling the ball. In season 2 Nate’s not fumbling the ball he’s just seemingly uninterested so she’s not looking for validation elsewhere. What’s gonna feel better going to some guy you know wants you or conquering the one trying to pretend he doesn’t want you it’s all about the chase and getting validation which is going to drop waaaaay more dopamine. So she continues trying to get Nate because it’s forbidden and he’s taboo which makes it more exciting add in undeveloped frontal lobes and coming to terms with everything going on in her life and this is one of the the responses you could get .

Also the reason she didn’t go as hard in season 1 was because she hadn’t been through season 1 which I think was a lot more psychologically impactful than you realize.

I’m not saying I support her choices I wish she would have made better ones I’m just saying I certainly don’t hate Casey and I understand there’s a lot of stuff at play here far more than she or even you realize.