Sashiko wrote:
1. Does not appear in at least the last three arcs: so not in books 9-13.
2. Does appear in the anime
3. The reappearance is unexpected
4. Does not appear in the Volume 14 illustrations
Sashiko
So we're looking for someone who only appeared in volumes 1-8 and were also in the anime...
There are 3 characters whose fates we never knew after volume 7 who would meet that criteria, Lu Gonghu, Chen Xiangshan, and Sekimoto Isao. Of those Sekimoto seems the most likely as he has a strong connection to the Thesis Competition.
I can't remember if Higaki Joseph survived the events in Okinawa but it would be cool to see him show up at some point. Interestingly enough there is a voice actor credit for him on Wiki but I don't remember seeing him in the anime.
Shiba Tatsurou and Sayuri haven't been featured recently but I would not classify their appearance as unexpected. Beyond them I can't really think of anyone else since so many characters appear in the Parasite Doll arc even if only briefly at the graduation ceremony.
I'll be interested to find out who it ends up being.