With Bella, I think it's less about being evasive and more about being asked to spill someone else's painful secrets. Jill definitely knows something about what happened with Bella's husband, but as she said she can't explain it without betraying Bella's confidence. To me, that's no different than Sage keeping the HOT secret code a secret.
Jill wanting to keep her family a secret is certainly a knock. Is it a bigger knock than Sage's (understandable) reluctance to discuss why she has such a hard time letting go of Chad? Debatable, but maybe a slight edge to Sage there.
I also think it's not quite true to say Jill keeps to herself. She spends a lot of time with Bella, even in the MC's presence. But it is definitely true that she and Bella are in a separate social circle from most of the rest of the game. (Another reason why her friendship with Josy and the upcoming D&G game could be so interesting.)
Personally, I think the biggest contrast between Jill and the other LIs is that Jill seems so much more comfortable in her own skin. Sage is the only one who comes close, but her obsession with Chad is eating away at her. As you said, it gets to the point where she is all but throwing herself at the MC just to get some much needed affection.
