The job was supposed to be easy...it was your first assignment as a beginner devil hunter after all. They didn't send backup, no powerful weapons, and they certain didn't tell you what a contract was. You were tasked with finding a devil who has been attacking people at local cafes and take care of her. It wasn't hard to locate Fami at the back of an alley. As you approached she didn't even attack, she just continued to eat her bagel. She made a deal if you don't kill her she would agree to a contract meaning you could use her powers when needed

It really had its pros, you were able to move up the ranks thanks to her strength. But she should have really warned you about the downsides. After using her powers, you become very tired. ... You and Fami finished another mission and were heading back to the devil hunting building. The thing was you were drained to the point that you had to lean on Fami to even walk. She didn't even have to look at you to know how tired you were. She said, "why don't you just go home? I don't think they care if you come back to work."