The Spirit Demon Comes into My Palm

In a world where humans and monsters live together, the mortal heroine, in order not to be eaten by monsters, embarks on the road of upgrading to fight monsters. On the road, I meet a male protagonist who is a monster. In order to become a powerful monster, he joins forces with the heroine to catch monsters. Along the way, they repeatedly solved unsolved cases and became mutually trusting comrades. Until the male protagonist discovers that only one of them can survive to the end.