This is bad writing. Obviously, the author has hinted at Yuri being the cause of all of this. From almost the first chapter she was introduced.
And that fits in with the cliche of a female lead character in a novel having basically zero female friends, maybe one. So in line with Yeoryeong being the female lead (I know it's Ham-I, but I'm explaining something else), all girls who act nice to Yeoryeong who are not Ham-I are just fake, manipulative, and bad people. That's the cliché, and the author would know this because this is a series about clichés.
But then the author goes full speed ahead? So far, the author doesn't even seem to be playing with this cliché, this trope like the author has with other tropes. There doesn't seem to be a deeper level or later to this. It's the cheap trope without more thought to it.
Maybe I'll be proven wrong. Maybe there's something deeper. I hope so.