shy; bashful; timid; reserved
彼女は内気で、みんなの前で話せません。
She is shy and can't speak in front of everyone.
内気な性格だったが、友達ができました。
Although I had a shy personality, I made friends.
息子は内気なので、学校が心配です。
My son is timid, so I'm worried about school.