Precocious

Also called as quick growth it is a special skill granted to the first human to kill a monster. Kudou Kazuto is the user of this skill. It allows him to level up, gain resistances, and some skills faster.

Skill description
The title is given to the first human who killed a monster in this new world. The new place created after two worlds merged. An enormous compensation is applied when earning experience. Attracting people, being led by fate and eventually becoming a being called a hero.