Every story has several basic "must have" requirements, such as, a hero(ine), a villain(ess), a plot, a beginning, and an ending. Along the way we pick up several other options, including other characters, different scenes, sub-thread plots and whatever else the author deems is necessary to tell the story.
A sidekick is not a secondary character. A sidekick is an important and critical aspect of the story. Where would Sherlock Holmes be without Dr. John Watson? Could Harry Potter managed to exist in several novels if Ron Weasley and Hermione Granger hadn't helped? Yes, Hermione Granger is a sidekick, if she'd been romantically involved with Harry Potter, then she would have been a love-interest, a totally different story element. Remember Robinson Crusoe? He had Friday to help him on the island. Even Frodo had Samwise Gamgee as a sidekick.
So exactly what does a sidekick do? Sometimes a character can stand alone, not need another to assist in the tale, but many well-known (as mentioned before) characters have had sidekicks to rely on. Why? They offer a different prospective and sometimes, even add a twist to the story by their actions. The sidekick's purpose is to enhance the characteristics of the hero or possibly offer a comic relief. Also, the sidekick will be the one who performs the less than desirable duties or those deemed below the lead's station. Yes, the sidekick is the grunt, the common laborer.
What are a sidekick's characteristics? Their flaws stand out, as do their physical features. Consider Ron Weasley: awkward, clumsy, red-haired, poor family. Sometimes the flaws or features aren't as obvious, such as Dr. Watson being a "retired" military doctor of great skills but those skills are advantageous for assisting the main character, such as Holmes, during his deductions.
Comic relief? Again, using Ron Weasley's inability to get it right, he often screws up an incantation with less than stellar results. Some of the results are quite comical and brings forth a smirk or giggle as one reads. This is especially true when the situation is straining with tension and Ron Weasley foils Harry Potter's attempt to perform.
Did I just state "foils Harry Potter's attempt to perform" in the prior sentence? Yes, another aspect of the sidekick is exactly that. Foiling the hero. How many times does a sidekick step forward to attempt their own form of justice only to be wrong causing either comic relief or total chaos of the hero's plans. I remember watching, as a child on that small, black and white televisions screen, the adventures of Roy Rogers, Sky King, The Ciso Kid and several other shows. They all had sidekicks. It never failed, Roy Rogers' friend, Pat Brady would be driving Nellybelle, the jeep, and Pat would bungle Roy's attempt to fix the situation. Even Nellybelle would pull a stunt to steal the scene. The same held true for the Cisco Kid with his sidekick, Pancho. I don't know how many times Pancho would try to be the hero, only to fail, foiling the Cisco Kid.
Yes, I love the sidekick character. This person can add much to a story: humor, tension, and a very much needed, second observation point.
How does one write a sidekick? What are the "rules" or guidelines?
Another aspect to consider: Villains do not have sidekicks. They can have minions or assistants, but a villain usually is a stand alone character. Why is this so, you ask? See rule number one - They are loyal and trust one another. A villain trusts no one and is only loyal to one person, themselves.
Does your next story need a sidekick? Only you can decide, but it might be a refreshing approach to consider.
And, as a special Christmas treat, I offer a short-story for your holiday reading enjoyment.