sidekick

Meanings

1. An assistant to another person, especially to a superior or more important person.
2. In literature, theatre, etc., a good foil of the protagonist, a character who helps emphasize the traits of the main character.

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.