Hazure Skill "Level Up" no Sei de Party wo Tsuihou sareta Shounen wa, Level wo Agete Butsuri de Naguru

At the age of 15, everyone is given a "skill." Chamber, who was a member of the party Akatsuki that was prominent in the city while being composed only of young members, was expelled from Akatsuki because he gained the skill "Level Up," a mystery skill with an unknown effect...! Although he was abandoned by the members who had acquired powerful skills, Chamber continues to work as a solo adventurer. In the midst of all this, he discovers the true value of the Level Up skill. "The more enemies you defeat, the higher your level and the stronger you become..." When he realizes that his skill was not a "failure" skill, he has a fateful encounter with a girl and spends his days in pursuit of strength...!

(Source: Kodansha, translated)