We are learning more and more about Sage.
Initially she came across as the bad girl, ready to fuck at a moments notice, anywhere, anytime. But it became obvious enough that she was hurting and the mc was just helping her to cope (sexually).
She's making mistakes as the President of the HOTs, trusting the self-serving Quinn too much, and too preoccupied with her own problems to see what is going on around her.
When she's finally out of the relationship with Chad, she's virtually throwing herself at the mc, even if he's shown no interest in her all along.
It seems Sage needs to be in a relationship to feel good about herself. Even a shitty relationship like the one with Chad, to her, was better than no relationship at all (hence why she stuck with him even when it was empty).
We find out she's not just some dumb bimbo but actually a hard working student, she just doesn't have the best of reputations (students and staff alike are surprised to see she's actually got brains - I guess when you prance around all day in a skirt that can never hide your underwear and your tits virtually hanging out of your top, people are gonna make some assumptions).
Then we find out she's not as kinky as we thought, and she doesn't know Quinn as well as she thought. The blindfold, the strappon, the ball gag, these all take Sage by surprise (while we didn't bat an eyelid).
And what do you know, the super sexy, guys would walk across burning coals to eat her shit, president of the HOTs, hasn't even had a threesome.
So Sage not being aware of Chad's gayness, Quinn's dodgy deals, her father's infidelity (and association with a campus prostitution ring), seems to be part of who she is. It's not that she's as naïve as Jill, it's just that she trusts those around her, and she's too caught up in her own shit to see warning signs (panties in Chad's gym bag was too much to miss though).
Regarding her never mentioning Tybalt as her brother, would you be telling people if he was yours?