Sung Jin-Woo is the weakest hunter (E-rank). It is embarrassing for him to even call himself a hunter. He is always getting hurt and has had a lot of near-death experiences. He isn't doing it because he wants to; he is doing it to pay his sick mother 's medical bills, which he can barely pay with what the hunter's guild pays him. For a person with no particular talent, he chose to become a hunter.
After accepting the quest he got the mysterious ability to increase his rank and strength like he is living inside a game, after that he quickly started powering up and tremendous changes in his strength and personality came with that.