Who was the girl
From what I can gather I think it's just his hallucination.
I'm a few too many years late in replying, but when you read the author's comments on the chapters, you can infer that she might be a ghost or something - the author wrote something along the lines that "maybe with such superior eyesight, they can see things that we can't" (in reference to that insect that was mentioned)
I like to read everything even the little comics and I can't read Japanese so it kinda pisses me off(╯°Д °)╯╧╧
╮(￣▽￣)╭
He was all.... oh no u aint f**king me!
http://www.mangago.zone/read-manga/joou_to_shitateya/uu/92c3f2b90892464d08938395c45e5c01_chapter-2/pg-25/
though he was in the end