Loyal companions or sidekicks tend to be many people's favorite characters. They often serve as a foil for the main character, depicting the good or bad qualities as they may exist. While there are varying degrees of sidekicks, the most loyal will protect the hero at all costs - even their lives if necessary. A companion is willing to do this because they believe in the cause, or the hero, or both, with such ferocity.
The sidekick character has undergone many transformations. There was a stereotype for a while of sidekicks being shown as weak, simpletons even, as an effort to show how heroic the hero was. But many would argue that while the hero's heroics are noble, the attention really should be on the companion who got them there.
There are many sidekicks who suggest an idea, have it dismissed by the hero as being ridiculous, only to have the hero seconds later make the same suggestions. We get a very keen awareness of that hero because of the sidekick. Watson keeps Sherlock grounded, Sam was clearly necessary for Frodo to have success, and without Hermione and Ron, one can assume Harry would have stumbled long before discovering the horcruxes.
There are many people, perhaps by nature of often feeling like less than heroes ourselves, who relate so well with the sidekicks that, when writing this kind of character, give all the personality and quirks and adoring qualities to the sidekick. In a recent Writing Excuses podcast, Brandon Sanderson pointed out how main characters have increasingly become flat. Sure, being around the sidekick can bring them out of their shell, help them develop, but if you have a side character in your story, take a step back from them and ask yourself if they are stealing the light from the hero. Our initial reaction is to say no, but perhaps a more in depth, honest examination will show a bit too much love being shared with a side character.
Do you have a favorite side character? Can you think of a main character who would not have made it without the side characters?