A sidekick is a superhero partnered with another in a lesser capacity. Used for comedy, to relate to readers, but also to move plot, and in some cases to become an integral part of the mythos.Or so says Comic Vine, which should know about sidekicks.
Sidekicks can often be as heroic as the people they are following. Take Ron Weasley in Harry Potter, or Amy Pond in Doctor Who.
They could be a younger person learning the ropes, like Robin to Batman. Robin eventually went on to become a superhero in his own right, as Nightwing.
Or the sidekick could be an essential part of the hero's armour, like Ando in Heroes. Without him, Hiro wouldn't have been able to get anywhere, because he couldn't drive, or to understand anyone in New York, because he didn't speak English.
For some reason most of my completed longer stories seem to be ensemble casts, but there are one or two which have had secondary characters - I don't think this is one of my strengths as a writer, and it's definitely something I need to work on more!
My main problem with secondary characters is that they either don't have any character at all, or I get too attached to them and end up giving them way to much background and character, then having to cut it all back out again when I realise they've become a main character and they shouldn't be.
Sidekicks are easier in some ways. Everyone loves it when Robin sometimes steps up to save the day. Sidekicks can have a developmental arc that is more dramatic than the hero's - they could go from a shy and scared kid to a brave teenager who helps to save the world. The key is to give them a bit of depth and not let them take over the story.
Yeah, I'm still working on that...!