Fight Monsters and Upgrade: I Signed a Contract with Hades

It's a cool story about upgrading urban alien monsters and fighting monsters. The protagonist is a young man who is afraid of death, but he embarks on this "fatal" career for money. He gradually fights against alien invasion by turning others into his own exclusive weapons, and also has to guard against ambushes from the evil forces of mankind. Later, he turns out to be a great hero who saves the world. This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.