sidekick

A sidekick is a slang expression for a close companion or colleague (not necessarily in fiction) who is, or is generally regarded as, subordinate to the one he or she 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 Puss in Boots, Mickey Mouse's Donald Duck and Goofy, Mario's Luigi and Yoshi, Sonic's Tails and Knuckles, Donkey Kong's Diddy Kong, Daffy Duck's Porky Pig and Batman's Robin.

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