subordinate but significant character in a fictional work

A sidekick is someone who regularly travels with, and helps, another person. In fiction, the other person is often a crimefighter or superhero, such as Batman and his sidekick Robin. Some people consider Sancho Panza, Don Quixote's ever-faithful companion, to be a type of sidekick, even though the book was written long before the word sidekick was invented. Annabeth Chase is nobody's sidekick.