Utsuro Naru Regalia

A girl is a dragon, and a boy is a dragon slayer in a world where the Japanese population has been eradicated. Giant dragons appeared over Tokyo, leading to the downfall of Japan. With the arrival of creatures known as Moju, humans vanished, and the country became a lawless region controlled by armed groups and criminal organizations. One of the few survivors, Yahiro, has become immortal through dragon blood and lives as a "salvager," retrieving artwork from the abandoned ruins of Tokyo. His routine changes when twin art dealers ask him to recover a particularly potent and enigmatic object.