This manga is very good. It is incredible that having only four volumes it manages in the first two chapters to create an intrigue that you want to know more about. Also the characters grow on you as if you knew them from many more previous chapters.
It is really well written, condensed into a short manga a complex and deep story with ease. (note that I didn't read the light novel in which it is based. In fact I just discover there is a light novel because of this manga).
The premise is quite interesting and original. And when you find out more about it it's become more intriguing. And the more practical and empirical way in which the characters approach it when digging around makes it even more interesting.
The plot is quite good, mixing somehow the school slice of life and the situations that usually occur in that kind of stories: love, friendship, bullying...(but not at all in a boring way and without taking time away from the more complex plot), and the personal stories of the characters with science fiction/supernatural.
The characters are interesting and charismatic. There are no usual tropes, nor anything ecchi just because. So it doesn't lose seriousness or credibility (I hate it when a serious story uses those nonsense resources).
The drawing is also simply fantastic: scenery, characters, facial expressions, very nice.
The general atmosphere and the vibe (the mystery, the drama, the friendship, the situations, the locations) reminds me of Summertime Rendering (but without violence or death) or the video game Oxenfree.
I recommend it to anyone who likes drama and mystery and wants a serious and well-written manga.