loyal retainer; loyal subject
Refers to a vassal or subject who is faithful to their lord or ruler, often in a historical or feudal context. Can also be used figuratively for a devoted follower.
He was a loyal retainer who served his lord.
Chushingura is a famous story.
As the saying goes, a loyal retainer does not serve two lords—he truly devoted his life to a single master.
As a loyal retainer, he served his lord faithfully to the very end.