Best Jeanist is a young man with light-colored hair. His mouth isn't visible because of his long collar which resembles jeans with a belt. In fact both his jacket and his pants seem to made of the same material as jeans pants. He also wears light-colored shoes.
Best Jeanist doesn't seem to like brutish people and he seems to be heroic as he wants to show Katsuki what makes someone a real hero.
Katsuki's workplace of choice is revealed to be Best Jeanist's hero office. In the office, Best Jeanist deduces that Katsuki chose him and his office because he ranks among the top five Heroes. Katsuki replies that he nominated him, to which Best Jeanist says he did since the other candidates have been good kids and it has been a while since someone like Katsuki has come along, telling Katsuki that reforming brutish people is what he does as a Hero. Best Jeanist tells Katsuki that he will show him what it is that makes people Heroes.