Scum of the Brave.

In the mid-21st century, Tokyo's mafia can turn into ether-enhanced Demon Lords, and Braves hunt them down. Yashiro, the Grim Reaper, reluctantly takes a new apprentice named Aki Jogamine, a high school girl training to become a Brave. He soon finds himself dragged into one troublesome case after another. The supernatural story of a scumbag master and his self-proclaimed apprentice begins!