he is op, knows he is op and that he could kill the demon king
in this world there are heroes who get marks on them to prove it, those people get a power boost and a role to go kill the demon kings
he trained so that he might get one of the marks to become a hero, kill a demon lord, marry a princess and live a lazy life of luxury but in a random town he overheard that kings had a shit ton of work so he started to live in a way to barely make ends meet while hiding his power so he wasn't forced to work (all of this in the first chapter)