So that leaves basically noble motivation. They want to do what good they can. They are loyal to their hero. Loyal enough with a strong enough friendship to be content with second tier and to be honest, that seems like the harder job.
One of my favorite examples of this is Samwise Gamgee. Totally the best character in LOTR. I took a class on Tolkien, and he thought so too. Sam doesn't get the glory, the hero status, he doesn't even have to go in the first place. He goes because of his friendship with Frodo and his love of the Shire. Simple, honest, true. And really, Ron and Hermione are just so much more engaging and less annoying than Harry.
I love imaging the back story of these side characters, and this is sort of my creative writing challenge for you all today. They don't have to be "side-kicks" in the comic book sense by any means, but just a side character from a story that you think might be interesting to look into. Here are some of my favorites:
Woodcutter in Red Riding Hood
Hunter in Snow White
Gepetto ( I love what Once Upon a Time has been doing with this character)
Anyway, what side character do you find interesting? If you were to write a story with that character as the protagonist, how would it go?