kindly in appearance but unfathomable at heart
He is kind in appearance but unpredictable in heart. It's the same as "deep feeling".
Idioms and allusions
Source: Qing Dynasty, Chen Menglei's "friendship breaking book": "swear to heaven, thick appearance and profound writing, enough to move."
Discrimination of words
Usage: as predicate, attribute; refers to the appearance of kindness, the heart is unpredictable.
kindly in appearance but unfathomable at heart
an important statement need not be prolix - yào yán bù fán
Carving the wind and carving the moon - diāo fēng lòu yuè
use one 's position to get even with another person for a private grudge - gōng bào sī chóu
The stars are all around the world - zhòng xīng huán jí
A man stands on his fist and a horse on his arm - quán tóu shàng lìdérén,gē bó shàng zǒudémǎ