A sidekick, according to Wikipedia,
is a stock character, a close companion who assists a partner in a superior position. Sancho Panza in Don Quixote, Doctor Watson in Sherlock Holmes and Batman‘s companion Robin are some well-known sidekicks….
Sidekicks also frequently serve as an emotional connection, especially when the hero is depicted as detached and distant, traits which would normally generate difficulty in making the hero likable. The sidekick is often the confidant who knows the main character better than anyone else and gives a convincing reason to like the hero. Although Sherlock Holmes was admittedly a difficult man to know, the friendship of Dr. Watson convinces the reader that Holmes is a good person….
While it is usually the reverse, it is not unheard of for a sidekick to be physically more conventionally attractive, charismatic, or physically capable than the character who is intended to be the hero. This is most typically encountered when the hero’s appeal is supposed to be intellect instead of sex appeal or physical prowess. Such characters are often middle aged or older and tend towards eccentricity; fictional sleuths and scientists for example. Such sidekicks are rarely encountered in fiction because the hero runs the risk of being upstaged by them. However, examples of successful such pairings include Inspector Morse and his sidekick DS Robbie Lewis, Nero Wolfe and his sidekick Archie Goodwin….
Other famous sidekicks — whose roles vis-a-vis their partners range from comic foil to friendly nemesis to voice of reason to stalwart ally — include (in no particular order):
I’m sure I’ve omitted other notable pairings. I’ll add them as they come to mind.