"A sidekick is a stock character, a close companion who assists a partner in a superior position. Don Quixote's Sancho Panza, Sherlock Holmes' Doctor Watson, and Batman's companion Robin are some well-known sidekicks in fiction.
The sidekick has the literary function of playing against the hero, often contrasting in skill or performing functions not suited to the hero. By asking questions of the hero, or giving the hero someone to talk to, the sidekick provides an opportunity for the author to provide exposition.
It may also be argued that the comedy sidekick's apparent stupidity makes a non-intellectual hero look intelligent. An openly flamboyant effeminate sidekick may make an unimposing hero look more masculine. A strong, silent and modest hero may have his fighting qualities revealed to the other characters and the audience by a talkative sidekick." http://en.wikipedia.org/wiki/Sidekick
So, get a beat-up cowboy hat, let your whiskers grow, mess up your hair, wear a tattered vest or coat, speak with a "twang" and say or do some goofy stuff to make the real cowboy hero look good, and you're probably a perfect cowboy "sidekick."
A good cowboy sidekick was George "Gabby" Hayes (top) who was sometimes Roy Rogers' sidekick "Windy Halliday." Walter Brennan (left) had some starring roles of his own, but he was often cast as a sidekick to others, such as John Wayne. Sometimes he was a bad guy.
My favorite was "Festus Haggan" on Gunsmoke, played by Ken Curtis for years. He was a perfect sidekick-- he rode a mule, not a horse. If he had to think about something, he said "I'm a studyin' on it." Perfect.
Another Roy Rogers sidekick was gravel-voiced Andy Devine, who played "Cookie."
Another favorite was Slim Pickens, who was particularly good in the 1973 movie "Pat Garrett and Billy the Kid" in the scene when he's dying to the music of Bob Dylan's song "Knockin' On Heaven's Door."
It's been said that the original cowboy sidekick was Lester "Smiley" Burnette, who played opposite Gene Autry. I don't remember him. He was just a little before my time.
It takes a lot of talent to be a good sidekick.