sidekick

Etymology

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.

noun

1. (informal) An assistant to another person, especially to a superior or more important person.
   In the abstract, Stuhlbarg’s twinkly-eyed sidekick suggests Joe Pesci in Lethal Weapon 2 by way of late-period Robin Williams with an alien twist, but Stuhlbarg makes a character that easily could have come across as precious into a surprisingly palatable, even charming man. May 24, 2012, Nathan Rabin, “Film: Reviews: Men In Black 3”, in The Onion AV Club
   If representation and recruitment is an objective, self-styled mavericks like Kelly and his Queensland sidekick George Christensen have some utility. 2 Feb 2021, Katharine Murphy, The Guardian
2. (authorship, figurative) In literature, theatre/theater, etc., a good foil of the protagonist, a character who helps emphasize the traits of the main character.