Mairimashita! Iruma-kun: Kalego Gaiden

Years before he became one of the most feared teachers at Babyls, Kalego Naberius was himself a student at the school for demons, dealing with a very big problem. For some reason, his schoolmates keep mistaking him for the shadowy figure picking off unruly students one by one—and trying to pay him back for the beatings. Not the type to sit and wait, Kalego and his classmate Balam start searching for the real culprit, only to find themselves in a cat-and-mouse game of demonic proportions…! (Source: Kodansha USA)