The sidekick in literature and movies/TV, while a minor character, can be as complex and as important to the story as the main character is. It is through the sidekick, or henchman if it’s a villain, we the audience can see a different side of the main character. Their job can also involve being someone the audience can relate to better than the hero.
Usually there is only sidekick. Think of Dr. John Watson, Gabrielle from the show Xena, or Robin from Batman. This makes it easier for the author and the audience to keep track of as well as providing a good balance. If you have a serious villain then there’s a good chance his or her henchman is going to be funny or a goof ball (see Harley Quinn to Joker in Batman: The Animated Series). Sidekicks don’t generally tend to be the same age as the hero or heroine, or the same gender either (I guess because some people think males and females can’t be just friends). Again this is to provide a contrast to the main character and to either bring wisdom, or not, and experience, or not, that the hero may not have.
Although keeping track of one sidekick is easy, it is possible to have more than one. For example, on Buffy the Vampire Slayer Buffy’s main sidekicks are Xander, who frequently provides the laughs and a brotherly figure, and Willow, who, once she becomes a powerful witch, provides back-up so Buffy can focus on handling the Big Bad while they handle the henchmen. In The Lord of the Rings trilogy, Frodo has Sam, Merry, and Pippin as his sidekicks even though Sam is Frodo’s best friend, Merry and Pippin come along for the journey. Modern literature and TV/movies experiment more with having multiple sidekicks yet it is still a rarity.
As there are different types of heroes and villains, there are different types of sidekicks exist. Some serve as a humorous foil. Others provide an alternate point of view, usually behind the scenes. Some possess skills the hero or villain doesn’t have or do tasks the main character can’t be bothered to do. Sidekicks can help expose flaws in the hero/villain and can act as a voice of reason.
However, a sidekick/henchman can also be a liability, sometimes distracting the main character from completing a specific, critical task. They can be kidnap and used as an Achilles’ heel as has been the case for some of the companions in Doctor Who. In the villain’s mind this forces the hero to choose between their sidekick’s life or defeating evil. Of course, the hero is always able to do both. Still, it makes for great drama and keeps an audience on the edge of their seat.
There are times a sidekick overshadows and surpasses the hero in popularity. For example, one could argue Spock is a more popular character then Captain Kirk even though Spock is second in command on the Enterprise. In this case it could be because Spock is so different from Kirk and a complete opposite to Kirk’s other sidekick Bones that viewers connect more with him despite his cool, logical personality. Spock, though, is the exception to the rule of when the sidekick becomes more popular than the hero does. Usually the type of sidekick that gains popularity is when they are the funny one as people like to laugh.
Naturally, not all main characters have or need a sidekick in order to be a fully realized character or to achieve their goal. Personally, I think the sidekick archetype needs to be used sparingly. While I appreciate a good supportive and funny sidekick, I’d rather focus my attention on the main character and his or her journey rather than getting distracted.