Huh... it seems we don't quite have a consensus on what exactly Hina is wearing.
- On one hand, we have the one-dress camp with the darker top blending as a pattern to the skirt.
- On the other hand, we have the two-piece camp with the darker top actually separate from the skirt.
- And on this hand, which I do think is very likely one by her portrait, we have an interpretation of a two-piece where the "edge" of the covering top part really is ragged.
There's no way the first one means a literal landmine, so I'm assuming it's a spin on the fact that being around her is literally dangerous – which is kinda like a "landmine woman", which seems to be basically a menhera hidden in the ground, a red flag hiding in plain sight.
Though a bandage around her arm instead of the ribbon does make me believe that this Hina might just benefit from therapy.
And the youkai pun there stems from 陽 sounding the same as the "you" in yōkai. And additional note that I'm guessing at this point that this contrasts with the "gloomy" 陰 – basically Hina's a socially awkward introvert.
And finally, there's a little nuance with the Ms. Kagiyama. The specific phrase is "Kagiyama-shi". The 氏 is very formal. Like if it's in English, a -san would just be a friendly, respectful tone. Using -shi here is equivalent to actually calling another person Miss/Mister like it's a conference or something. The idea is that Hina awkwardly called Nitori "Ms. Kawashiro" and Nitori genuinely replied back with "Ms. Kagiyama" – and they've been stuck with that ever since.
