tough at heart; strong-willed inside
Describes a person who appears gentle or soft on the outside but is resolute and strong on the inside. Often used in contrast to 外柔 (がいじゅう, outwardly gentle).
He is tough at heart, and despite his appearance, he has a strong core.
There is a saying: 'gentle on the outside, tough on the inside.'
gentle outside, tough inside; iron hand in a velvet glove
gentle but firm; soft on the outside, hard on the inside
Compound of 内 (inside) and 剛 (strong, firm). The exact historical derivation is uncertain, but it is typically encountered in the idiom 外柔内剛.