Shadow and Déaspor. While they are heroic, their repeated Jerkass behaviour is left unexplained and subject to interpretation.
Just how happy Bridget's posse was to go along with her is unclear, as she is keeping them in line by bullying. One girl in her posse, Lola, seems more cooperative than the other girls with Bridget, but after the Snowball Fight, she is the only one among them who is explicitly mentioned as actively trying to befriend Lu; this seems to imply she's a Turn Coat.
Lu is surprisingly calm when she's about to die. Justified, given her self-loathing.
Lu: Goodbye, world.
Lu tells Cleareye about her experiences in the Dark Ones' tunnels with surprising composure. This is lampshaded.
Evil Is Cool: The Dark Ones avert this. While their Throne Hall sports a spectacular design, and their king is an accomplished scholar (see the Wicked Cultured entry in the Literature tab), they barely manage to evoke any sympathy.
Lu mentions how cold the weather is as an offhanded remark, despite it being the coldest winter in 100 years. Which makes perfect sense — she just got there from Scotland, where the weather is generally colder, and she barely talks to people around her (who, presumably, would want to Talk About the Weather), so she doesn’t give much thought to how abnormally cold it is.
The Foreshadowing using Chekhov's Gun seem to be a bit heavy-handed and spelled out until you realise it makes perfect sense in-universe — "Hat Lad" and Déaspor want to make sure Lu was following The Plan exactly as she is told, even if she doesn't know why. A clear example would be Déaspor giving Lu the map at the Chopped Tree Inn: she knew she was going to meet Hallwad, who was also chosen to play a part in Shadow and Déaspor's plans, and was practically spelling it out for her to show it to him.
The poem recited at the Dark Ones' feast mentions by name only three characters and one place name (and a demonym, if you're reading the Hebrew version), but Lu still says she has trouble keeping up with the names after only Roythebrune and Orce-Blatt are mentioned, and she was already familiar with Orce-Blatt. So what's there to mix up? It makes sense when you notice she mentions by name pretty much everyone she interacts with, so she's struggling to keep remembering the new names in general, not just in this poem.
Rewatch Bonus: Hallwad hesitates before answering when Lu addresses the Arc Words at him. Re-reading the scene after discovering they're the traditional Killer blessing explains why: he was worried that she might be a killer after all.
Slow-Paced Beginning: The first third or so of the book comes off mostly as a typical teen drama about an underdog outcast and her conflict with the resident Alpha Bitch, with some fantastic elements thrown in, albeit with a few funny quips by Lu, and Professor Cole's genuinely terrifying lashing out at Lu, followed by a gut-wrenching Heroic BSoD scene. Once she crosses over to Greywall'd, her attempts at survival and her struggling with her heritage and nature make the book far more engaging.
Woolseyism: The translator, N. L. Lumi, is a native speaker of both Hebrew and English who's had experience with translating since elementary school, and has written some poetry on commission. Feeling that Lu's language should be richer after she becomes an Omniglot, they embellished the language of the original, which used somewhat typical teen speech patterns, every now and then. The four poems featured in the book — the poem at the entrance to the Dark Ones' tunnels, the poem their cantor recites at the feast, the song from Lu's vision, and the song Cleareye sings to Lu — showcase their talent in particular.
. Which makes perfect sense — she just got there from Scotland, where the weather is generally colder, and she barely talks to people around her (who, presumably, would want to Talk About the Weather), so she doesn’t give much thought to how abnormally cold it is.