I’m going part literal and part metaphorical with the shadows with vols. 13-14 of Toilet-bound Hanako-kun. On the literal side, a good chunk of the story in these two parts of the series take place in the spirit world which seems like a shadowy place, and the spirits who can cross into the real world are almost always popping up out of a shadow. This is also the part of the story where two shadows (ie mysteries) that have been going on for a while in the series start getting some attention.
For a long time Aoi has been main-character Nene’s friend at school; other than the popular prettiness kind of envied by Nene, there hasn’t been much to her. Once she and the guy who openly as a crush on her end up in the spirit world, heading towards the afterlife, he calls her out and the result is a whole lot of character possibility we haven’t seen before; granted the spirit who is responsible for here they are seems to have messed with her head a bit, but Aoi suddenly has some depths she didn’t quite have before. Who Aoi is has been part of the overall mystery since even Nene wonders a time or two what Aoi’s connection to the spirit/ghost world might be. We both find out some of that, and what it might have to do with a big plot challenge (Nene’s apparently impending demise).
There’s some further mysterious, ghostly shadowy things going on with the sort of cliff-hanger at the end of volume 15 too. On one hand, if you know your Greco-Roman mythology, you’ll recognize the plot twist suggested when one of the groups who ended up in the spirit world returns to the real world (probably); what makes it possibly interesting is that the Hanako-Nene group also make it back, but Hanako has a solo moment suggesting additional though related impending something. Nene has hope for maybe solving her life-span forecast now, but Hanako’s observation suggests that the solution underway may not be something she’s going to like when (obvs it’s when not if) she finds out.