Hollow Regalia, Vol. 1 (light novel): Corpse Reviver — Gakuto Mikumo

Four years ago, a dragon filled the sky, the nation of Japan fell, and Japanese people around the world were wiped out. Yahiro, a survivor of the tragic event, works as a salvager who obtains Japanese artwork and artifacts for foreign buyers. Not many are willing or able to brave the ruins, which are crawling with monstrous beasts called Moujuu. But Yahiro's the best man for the task. After all, thanks to his regenerative abilities, he can't really die. He's approached about a job to search for the Kushinada, which is capable of controlling the Moujuu, but it will require delving into the most dangerous parts of the city. This is the story of a dragon girl and a dragon slayer.