Cristo - Jougan no Messiah

Towa became a sweeper admiring the legendary sweeper called, “Orange-Eyed Messiah”. The task of sweeper, which requires to “sweep” the monsters Adys is quite hazardous. On his first real attempt at fighting Adys, Towa encountered Kei, a former sweeper who saved him from being killed.
Towa asked Kei to take him as a student. However, not only Kei plainly rejects Towa’s request, he tells Towa that he saw the “Orange-eyed Messiah” die!? An appearance by a mysterious man who seems to be connected to Kei showed up with a grudge from the past. Who is he and what is his relationship with the “Orange-Eyed Messiah”? Will Towa finds out about Kei’s past too?