Dark Gathering, Vol. 5 (manga)

Having captured the spirit of the H Castle Ruins, Keitaro, Eiko, and Yayoi next set their sights on Tokyo’s infamous Old F Tunnel. But before they can brave their first Danger Rank S location, Yayoi takes them on a detour to collect another one of her Graduates - the ghost of a soldier who, even in death, could not escape the pain of war.