LV999 no Murabito

Monsters did not always exist in the island country of Hexaldoria, once known as Japan. But, once monsters appeared, their numbers grew in an instant. Eighty percent of the world is now too dangerous for people to live in. With the monsters' arrival, the world bestowed "Classes" upon its people. Classes are assigned at birth, and most people live according to them. There are eight competent classes: Fighter, Hunter, Cleric, Warrior, Mage, Merchant, Thief. And three special Oracle classes: King, Sage, Hero. The weakest Class of all is Villager. However, Kouji Kagami became "Level 999"--the highest level that no one had reached before. When he met the daughter of the Demon King, the world started to change!

(Source: Kadokawa, edited)