Ah, sidekicks - the usually rational, sensible voices behind the heroes. What a thankless job that is. Witness poor (Not in the face, not in the face!) Arthur of The Tick fame. While the heroes gather in a swanky bar to swill free, fancy drinks and tell tall tails, the sidekicks are relegated to a tiny, sweltering shed in the back that serves flat soda. The wages of common sense and a certain flair for hauling your best friend's backside out of potentially deadly situations.
The problem is that sidekicks are almost entirely a literary invention. Sidekicks don't happen in real life. Don't look at me like that. You only *think* I'm your sidekick. In my mind, I'm the star of my daily life and you're the supporting actor. That's human psychology. We're each of us the center of our own universes. Our friends fill whatever roles we have mentally scripted (The Funny One, The Smart One, etc). But that means each of us features in the supporting roles our friends have created. This works out. I'd like to hang a little more with those of you who have me cast as "The Smart One". Though, I may be about to undo that assessment.
Beyond humanizing the hero, mentoring the hero, and so forth, from a psychological standpoint, the sidekick *is* an aspect of the hero's psyche personified. Arthur, the sidekick mentioned above, really is the quiet (and largely ignored) voice of reason sounding in the Tick's head. He is the Tick's disowned grasp on reality and the mundane. Tonto? Represents the Lone Ranger's instincts and his connection to something larger than himself. Chewbacca? Represents Han Solo's ferocious, animal nature - not that he doesn't do ferocious reasonably well on his own, but the fact that he hangs with this creature who's been known to rip his opponents arms off when he loses a game...yeah.
One of the other reasons I feel sidekicks are a lie is that, in my opinion, they're only sidekicks because they haven't yet faced the challenge that would turn them into heroes or heroines in their own right. Every character in a supporting role in a story has the potential to become the next book's hero/heroine. All it takes it the right fire to temper the different mettles.