sidekick

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.[1][2]

• IPA^(key): /ˈsaɪdkɪk/

sidekick (plural sidekicks)

1. (informal) An assistant to another person, especially to a superior or more important person.
   • 2012 May 24, Nathan Rabin, “Film: Reviews: Men In Black 3”, in The Onion AV Club‎:

     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.

   • 2021 February 2, Katharine Murphy, The Guardian‎:

     If representation and recruitment is an objective, self-styled mavericks like Kelly and his Queensland sidekick George Christensen have some utility.

2. (authorship, figuratively) In literature, theatre/theater, etc., a good foil of the protagonist, a character who helps emphasize the traits of the main character.
   • 2023 October 29, Zoe Williams, “‘An ironic, self-deprecating metrosexual’: how Matthew Perry captured the spirit of the age”, in The Guardian‎, →ISSN:

     So even though he had – no question – the best lines in Friends, he was never what he would have been in an earlier era: the sidekick.

• acolyte
• deuteragonist
• pupil
• wingman

• mentor