A sidekick is defined as any character who has worked alongside, or apprenticed under a more experienced mentor. The only exception to this rule is Pat Dugan, the hero known as S.T.R.I.P.E. Dugan was once the sidekick to the original Star-Spangled Kid, a character several years younger than him. Later, he became the sidekick to Courtney Whitmore, the hero known as Stargirl - again, a character who is many years his junior. See Sidekicks for a complete list of known sidekick characters.

A sidekick is a stock character, a close companion who assists a partner in a superior position. Sancho Panza in Don Quixote, Dr. Watson in Sherlock Holmes, and Robin, Batman's companion, are some well-known sidekicks.