sidekick
n. countablen. a person who helps and travels with someone more important or powerful. You often see them in movies or comics as the hero's loyal friend.
n. a person who functions as a subordinate companion or assistant to a primary figure. Often implies a relationship of loyalty and shared adventure, frequently appearing in narrative fiction.
The hero and his sidekick saved the city from the villain.
Every great detective in fiction seems to have a loyal sidekick to help solve the most difficult cases.
While the protagonist receives most of the glory, the narrative often relies on the sidekick to provide necessary technical support or moral grounding during the journey.
From side + kick, which in the late 19th and early 20th century was a slang term for the front side pocket of a pair of trousers, known as the pocket safest from theft. Thus, by analogy, a "side-kick" was a person's closest companion.
Informal in register; commonly used in the context of superheroes, detectives, or close friendships.