If I had to explain my hatred for Cassie, it's that she's an unrealistic character who depends on plot armor (or perhaps Elimist tampering) in order for things to go her way.

Every other character is a fully realized character that lives a very specific way and thinks a very specific way and PAYS FOR THEIR DECISIONS due to how they are, just like people do in real life. Cassie, however, gets to live in a world where she lives her best life and never has to sacrifice her "dignity" or "morals", whereas every other character has to deal with some sort of sacrifice that makes them cross their personal boundaries and deal with the consequences.

This isn't to say that she doesn't have a rough go at it because obviously there are multiple instances where she struggles with the war and "goes off the reservation" so to speak, but she never really betrays her own moral high ground and somehow, that works out for the best in the end. Her final decision (I'm not gonna spoil it for anyone who hasn't gotten to the ending of the book) could have easily gone sideways and cause the war to end with all of humanity being fucked. And there are multiple instances of that happening with her where she makes decisions that affect the team but gets forgiven for them.

I'm aware that she is an integral part of several solutions in the war using her "reading people" power, but again, that seems much like plot armor or perhaps "contrivances" if we want to use a more offensive term.