How we cite our quotes: (Chapter.Paragraph)
Quote #4
A human marked with the devil's eyes. Why? There was only one possible answer, as plain as it was disturbing. That [Karou] was not, in fact, human. (14.75-14.77)
This is the first hint that Karou isn't human. She doesn't know it yet, though. Since Akiva's not human either, maybe it takes a non-human to know one?
Quote #5
"What are you?" [Kaz] asked. Not who, but what. (19.41-19.42)
This is a pretty biting question, directed at Karou while she's smack-dab in the middle of her identity crisis. At this point, she has no idea who she is, and being called a what makes her wonder if she's really a monster after all. Poor Karou.
Quote #6
[Karou's] name, called out to her, didn't always register, and even the lay of her shadow could strike her as foreign. (22.6)
The more lost and confused Karou becomes after Brimstone's disappearance, the more disassociated from herself she becomes. As we've heard, a shadow tells the truth, so the fact that her shadow is foreign tells us that she's really someone else.