Sidekick (nonfiction)

A sidekick is a slang expression for a close companion or colleague (not necessarily in fiction) who is actually, or generally regarded as, subordinate to the one he accompanies.

Some well-known fictional sidekicks are Don Quixote's Sancho Panza, Sherlock Holmes' Doctor Watson, The Lone Ranger's Tonto, The Green Hornet's Kato, Shrek's Donkey and Batman's Robin.