Heroes need sidekicks. Sancho. Friar Tuck. Robin. Sam Gamgee. Herminone and Ron. Rocky. Agent 99. The sidekick is often allowed a more extreme version of personality than the hero; like Donkey is to Shrek.
Sometimes the sidekick is a good device for narration, like Doctor Watson supplying the reader’s POV. Sometimes the sidekick is there to rescue the hero in the nick of time, like R2-D2 (photo compliant with Rule of Sexy, may not be authorized by Lucasfilm) does for Luke Skywalker. But a good story about a good hero almost always includes a sidekick. It goes back to the model of the Hero’s Journey, in which the hero befriends another journeyer, who helps the hero overcome a struggle along the way to the ultimate conflict.
The differentiation between sidekick and hero is that the narrative focus is always on the hero. If you have partners (say, Laverne and Shirley), they are narratively equal. A good sidekick, though, leaves you wanting more, even though in the cool light of recollection you probably realize that the ratio of hero-to-sidekick is never going to be quite enough to satisfy you.
At the end of the day, I’m going to say my favorite sidekick of all time is Inigo Montoya. A badass in his own right, switched from enemy to friend of the hero through earning mutual respect, was driven by a simple and powerful motive, carries the hero through a difficult portion of his journey (the hero was mostly dead, after all), and he got two of the best sidekick lines ever.
Think you you do better than Inigo? That’s what the comments are for!