I'm a librarian who loves anime, manga, and reading a wide variety of genres.
Aww. Ennis is a homunculus created by a guy named Szilard. He gave her the minimum knowledge necessary to serve and protect him, but then she obtained some extra knowledge and experiences and developed a conscience. Specifically, she suddenly realized she'd done horrible things for a horrible man and would have to keep doing horrible things if she wanted to continue to exist.
I think Ennis might be my favorite character so far, although the way Isaac and Miria play off of each other is fun. I don't recall liking Ennis as much in the TV series, so the book definitely seems to be adding something to the story for me.