He is Tonto. He is Robin. He is Morningglum. He is Samwise Gamgee.
He can be a woman, but only if the Hero is a woman. Heroes don’t have co-ed sidekicks.
He is either younger than the Hero or of a lower social caste (or sometimes both). The sidekick can not be the Hero’s social better.
The Sidekick loves the Hero. That love is never sexual, but it often has a sexual intensity. The Sidekick, never the less, is not gay. His relationship with the Hero is always chaste.
If the Hero ever did want to have sex with the Sidekick, he probably would say yes but, since the Hero won’t, the point is moot.
Sidekicks can have girlfriends (and usually will) and even wives, but they can not have a One True Love. Their primary love will always be the Hero.
A Sidekick can be heroic. A Sidekick can even Save the Day. The Sidekick, however, will never Complete the Quest, Defeat the Villain, or Get the Girl. Those accomplishments are reserved to the Hero.
The Sidekick’s main responsibility is to Take the Bullet. Once the sidekick has done this, the Hero will proceed to avenge him.
The Hero is the Mentor of the Sidekick. The Hero can teach the Sidekick but never the other way around.
A Sidekick can eventually grow up and go out into the world to have his own adventures, but he will always be in the shadow of the True Hero.
The Sidekick can betray the Hero through his own weakness. If he does, he will regret it and will have to redeem himself to the Hero.
The Sidekick wants to be a Hero but he is not jealous of the Hero. At most, he wishes that he could live up to the Hero’s example.
The Sidekick can provide Comic Relief. Although the Hero can be embarrassed (often by the Sidekick), the Hero will never be mistaken for a clown. This is not true of the Sidekick.
The Sidekick will try to comfort the Hero in times of distress but he can never be the one to resolve the Hero’s anguish.
The Sidekick would avenge the Hero if he could, but he never will be required to. At most he’ll be under the mistaken belief that the Hero needs to be avenged. In the end, though, the Hero must provide his own vengeance.
The Sidekick may carry the Hero if the Hero is wounded, but never all the way.
The Sidekick usually believes in God, but won't say anything unless the Hero asks him. The bulk of his worship is reserved for the Hero.