Nagi is the trype of character that i love the most. Being in the middle of "Perfect in every way(impossible) and "Perfect in every way, but kinda possible", it's mysterious and exciting as its best. Mary Poppins has the same type, if i'm not mistaken(ok, you can't control time and space like Poppins, but what you can do is being a perfect Nanny).
Mary Poppins is already "Perfect in every way", she herself says it in her first movie(1964), but it's mysterious because you don't really know what exactly she is. Is she a normal human with powers? Is she a demi-god? an entity? no one knows, and it just makes it more interesting.
The mystery behind Nagi is that she's just... creepy. You don't know what she can do, and god, it's exciting. She could 100% be ACTUALLY dangerous(like KILLING someone to protect Chihiro), but there's no way you'd though about that, since she's so perfect. You can't really tell what Poppins can do for her kids either, she already "created" a whole universe in seconds, just like that, for the sake of "teaching" something, what more she can do?
Ok, I got worked up lol
Also srry if I said something wrong, english isn't my native language :p