These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The cover of the book is pretty unnerving once you look at it twice.
After the reveal, Emily becomes frightening.
The wordless illustrations on pages 154-155 that show a crying, angry Emily burning down the house of her crush and his girlfriend. Her expression is what really seals it.
Emily's picture in the newspaper is terrifying. Maybe it's just because it's the first time she's seen with pupils, or just as a normal person, but the dark circles and murderous expression kind of just hits you.
Viewer Gender Confusion: Apart from the obscure feminine name and a couple fleeting snippets of dialogue, it can be hard for some readers to tell that Siobhan's a girl. It doesn't help that she wears a necktie and the male school uniform.
The Woobie: YMMV, but arguably Emily - with the whole backstory of being murdered and stuck in a well as a ghost for ninety years. ...but then, of course, we find out she was lying about her past
Emily might be more of a Jerkass Woobie if anything. Anya may technically count too.