[Used Can]

^It seems more like there are 2 side volumes... one named "lost girl with cat" and the other one named "two dogs were left".

I suppose both side stories will focus on showing us the stories of Suzuka and Ohtomo respectively, before the series started.
Going by the covers and the titles, it also seems both Suzuka and Ohtomo will get animal companions - a cat and a dog respectively.

I wouldn't be surprised if Suzuka's volume ends with her reaching the decision of reviving her brother, or her heading toward Harutora's town, and if Ohtomo's volume ends with him being offered a teacher position at the Onmyou Academy.