One's true character or true worth—the real personality or nature beneath the surface.
長年一緒に働いて、彼の真面目がやっと分かった。
After working together for many years, I finally understood his true character.
面接で見せたのは演技ではなく、彼女の真面目だった。
What she showed in the interview wasn't an act but her true self.
友人は普段は冗談が多いが、困ったときに見せる真面目に信頼できる。
My friend jokes a lot normally, but I can trust the seriousness he shows when in trouble.
Serious; earnest. (Historic/alternative form — in modern Japanese usually written as まじめ.)
彼は仕事に対して非常に真面目で、手を抜くことがない。
He is extremely serious about his work and never cuts corners.
両親は子供に真面目に取り組むことを教えた。
The parents taught their child to apply himself earnestly.
古い文章では『真面目』の表記が見られるが、現代では仮名交じりで書かれることが多い。
In older texts the spelling 真面目 can be seen, but in modern usage it's often written in kana or mixed script.