And i would be okay if that how she was written read my feedback would, i have problem with writers repeatedly making point of regarding her and her father where she cant even perform the human sacriface shit because the slave looks like her father only to literally gut him like fish in chapter 5, without any opportunity for you to interfere there, thats where it gets pozzed, notice how with other companions romance or not you get to influence them Ember Has several different endings from being freaking Child Crusade on her own to becoming apathetic and no longer caring about people etc while she gets railroaded, the point here she is written as fucked up monster but still human and then goes writers go full Ryan Johnson, its just bad writing. Like if they gave player way to influence her with her "good" ending basically ending with her still massmurdering people but trying to pick her targets more in Dexter like fashion yeah i accept that, but its the lack of actual player influence there, not her being twist on character dynamics.
Her not changing like other characters suits her style, though. She never really gave a shit about the entire crusade, or other people besides herself. She grew up that way. Everyone just happens to be a tool for her own purpose. Nothing about that changes, the environment doesn't change, if anything, experiencing the abyss and the levels of torture and pain and violence inflicted there would probably only appeal to someone like her and make her crave it more once she returns to the "human" world.
Not everyone deserves a happy ending, and typically its the characters that don't get one that are the most consistent and make the most sense. I doubt people would care this much if she wasnt initially portrayed as a potential romance option and the stereotypical cute half-elf one at that. I also bet you that most people would wanna romance Ember despite her being essentially an autistic 9y old child. But the game made the effort to have the characters act independently, which allows for a wider choice of characters, and makes them disappear or re-appear at certain moments throughout the game. It's why I think the cast of companions is pretty solid, especially for an Evil playthrough, which tend to lack good companion options. Having companions stick to their convictions despite whats happening around them (or exactly because of it) shows strength of character, and I personally enjoy that Regill doesn't lose his rigorous military attitude or that Cam doesn't stop being a murdering sociopath. Some characters you can influence and your choices affect their development, but having every character fall within that category seems pretty damn boring to me.