one's subordinate; one's henchman
Idiomatic expression referring to a person who works under someone else, often with a nuance of loyalty or being a trusted follower. Can carry a slightly old-fashioned or dramatic tone.
As the president's trusted subordinate, he is entrusted with important tasks.
That man is a henchman of the gang leader.
Literally 'person of the hand', suggesting someone directly under one's control or at hand. The exact historical derivation is uncertain, but the expression is idiomatic and has been used in Japanese for centuries.