I cant speak for others but as readers who are women it is fully right to hate characters like yiyi. Why? Because she represents nothing about real women-not even 1%. She is just a creation & addition of male fantasy/gaze merely for male fanservice.
A case is point--
--she is supposedly a hunter (so u have these isolated panels with her taking down 1-2 demons to show readers that she is a hunter. But the reader barely knows her hunter skills or how significant a contribution she makes by herself on the battlefield because it's all about the heroic male show all the goddamn time)
--she is barely clothed, enticingly so that she looks young & attractive enough (after all no plain Jane soldier girl in neutral gear will do for male fanservice)
--she is not a poor person (even male's fantasies are about wealthy women who will pander over them)
--her grandfather is powerful (for some reason Chinese male fantasises incorporate powerful families for the female lead--- again no waitress girl will do)
These are just some reasons why female characters like yiyi are hated. Not because she is female but because there is nothing real about her.
Characters like her implicitly by their very existence tell the female readers that this is what females must become to be desired.
She is not expected to be a person or human. She is to remain a doll, a canvas...upon which male fantasy will paint & create.
Like how Sylvia Plath finally put her own head in the oven from the pressures & rigours of male expectation on the female gender--most women are driven to that point of mental discomfort when given such bald-faced vulgar caricature to imitate!
I hope u understand. I didn't lay out the points above to argue, but to try to make u see the frustrations of a female reader & why some readers are driven to hatred of such characters like yiyi. (Just to let u know, im quite sure the rest of the other female readers either react with disgust or just resignation)
As a female read, all I can say is WOW you are mad that a female character in a comic targeting men... is attractive to men? She is no different than a male character in a women centered romance, a rich attractive thing for the reader to salivate over. Do you get mad at romance genres for women for doing this? Again, Yiyi has done NOTHING for the hate. I can understand this hate if the love interest was the female co-student from his high school, who refers to him as "dumb" consistently, because let's face it in any other comic she would be the love interest. But Yiyi has just been a girl, existing for the male gaze, oh no!! I can name dozens of romance comics with male characters written exactly the same way. Again, I'm a female reader, I'm not disgusted by her, or resigned to her existence. If your explanation is the reason you don't like her, I say that's more a YOU issue, than an issue with the comic.
Like for you to dislike her is all on you but as a woman myself, If you don't like her ignore her then. She's not weak, Mc is just OP and likes her enough to fight for her. I feel like she a rare FL where she isn't in the way and yes she's rich and well off (As there are women who are rich and well off in the real world) but that doesn't take from the fact that she herself is not weak and doesn't flaunt what she has and just exists which I love that for her. She isn't what you want from a FL which is fine but to hate her is a bit of a drag especially since unlike a lot of the other fl in these type of story she isn't a burden and can at times be helpful with her connection and resources.
The pacing is a bit chaotic.
Also, why all the Yiyi hate? She hasn't done anything to deserve it.