na-adjective, noun
overserious; deeply sincere; very earnest
Describes a person or attitude that is extremely serious, earnest, and sincere, sometimes to a fault. Can carry a slightly negative nuance of being too stiff or humorless.
彼は生真面目な性格で、冗談が通じない。
He has an overserious personality and doesn't get jokes.
生真面目に仕事に取り組
She tackles her work with deep sincerity.