Definitely some weirdness here. Some of it is in the plot itself, but I gotta give it a thorough reread to make sure I didn't mess up anything on my end.
Well, I'm confused. Seems to be a pretty complicated read. Some dialogue balloons not being clear on whose it is in an empty panel kinda exacerbated it.
Hmm... Yeah, still confused. Armchair theories welcome.
Right now, my best guess is that Koishi (through an unusual interpretation of her ability) caused the man Tatsuho to think he saw a doppelganger of himself... which caused him to rent out the "Shadow Sickness" book which was a youma book containing an actual doppelganger.
Don't take my word for it, though. There's also a decent chance that my confusion about the plot caused me to mistranslate some ambiguous bubble and pass it forward... or maybe the plot really just is weird.
I'm late here, but I imagine the point is that Koishi is showing up as the person that the viewer unconsciously would expect to see in that situation. Akyuu often visits the book store, a relative of the man's at his house, a doppelganger of herself once the mailman informs her of the doppelganger story (and his loner status,) then Keine when Kosuzu needs to be saved. As for why the man saw himself in Koishi, don't think we're even given enough information on him to really know why. Perhaps he rented the book BEFORE seeing Koishi, causing him to imagine her as a doppelganger, or maybe he's just heard of the legend before
Greetings, Yanokura here! ( ◎ Д ◎)
Thank you very much for picking up this book.
The electric stove called it quits in my cold room in the middle of crunch time.
This is my third Kosuzu book, but I'm really sorry about it not having enough AkyuSuzu stuff. It really is hard to stuff in a ship without disrupting the story, huh?
I mean, I know it's a doujinshi so "Don't worry about it, just let them smooch!", but still...
I'm anxiously looking forward to Forbidden Scrollery Vol. 2 coming out.
After this I feel kind of spent on writing Kosuzu, so I kind of want to try writing a different character next time.
Well, see you then. Let's meet somewhere once more.