Isabella coming to Frey's rescue when he was bullied is irrelevant because you're right she doesn't have to do that. My issue is that her sudden attraction from him comes out of nowhere because they were classmates before Frey commited suicide. It comes from left field because Frey's apperance doesnt when Lucus possesses his body. Everyone in the school new about Frey because he was the weakest trainee, so its practically impossible that she didnt knoe who he was.
I totally get where you coming from, but if even her having the bare minimum interaction with Frey would've been enough for me. Like her saying hi to him every once in a while or at least acknowledging his existence would've been nice and I really wouldn't have been so resistant to them getting together. Its just so forced ╥﹏╥

I dont hate Isabella, in fact I kinda like her. I dont think it's wrong of her to feel attracted to Fray after he suddenly became strong since that's a typical clinch reaction. Moreover, if you think about it's a dog eat dog world. And if she had helped the former fray what then?
Does she always have to come to his rescue each time he gets bully? Wouldn't his bullies do more attempts to kill Fray is Isabella had helped him? Wouldn't helping him bring his self esteem down since she a "woman"?
Also, how would her Household react if she had helped him? Would Fray's father even react positively that he received help from a woman? Or be suspicious of him for getting help from someone of her household?
Theres like a lot of social and political problem involve with helping the victim of the bully that's why people of by standards find it hard to interfere or think of what will benefit them in return for helping when they could be at risk as well. It's a shitty society but that's the real world peeps. Instead of making it unrealistic like usual stories goes, I like how this one goes.
Hate all ya want, or like all ya want. I personally like Isabella. I have said my long piece.