stern face, kind heart
Describes a person who appears strict or intimidating but is actually compassionate and gentle inside. A yojijukugo (four-character idiom).
He has a stern face but a kind heart; though he looks strict, he is actually very gentle.
A yojijukugo combining 鬼面 (demon face) and 仏心 (Buddha heart), literally meaning 'demon face, Buddha heart'. The exact origin is uncertain, but it follows a common pattern of contrasting outward appearance with inner nature.