Sidekick

wiktionary

ref

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, was known as the pocket safest from theft. Thus, by analogy, a "side-kick" was a person's closest companion. [1] [2]

etymonline

ref

sidekick (n.)

also side-kick, "companion or close associate," 1901, also side-kicker (1903), American English, of unknown origin. Earlier terms were side-pal (1886), side-partner (1886).