The friends and helpers of the main hero. They can be almost any type of hero playing a secondary role, a normal character observing the action, or Plucky Comic Relief - sometimes all three. Typically they are a Foil to the hero, and this is often underscored by their dramatically different appearance.
Sidekicks often get themselves into trouble, so the hero can rescue them. They also give the hero or Mr. Exposition someone to explain the plot to. In the finale, they may line up against the Evil Minions. (The reason why they are called sidekicks and not the Good Minions is part tradition and inertia, and part the differences between the two relationships — because the hero and sidekick both are heroic, reciprocal bonds of loyalty and friendship are far more likely than between the villain and his minions.)
Sidekicks are almost never older than the main hero.
In Comic Books, Sidekick Graduations Stick: the "graduation" of a former sidekick into an independent hero in his own right is one of the few categories of changes that tend to avoid being reversed in the long run. However, before that graduation you'll see them get stuck with a lot of dead ends, like Hero Harasses Helpers and the Sidekick Glass Ceiling, though he rarely gets hit with Never Be a Hero. But if the hero bites the dust or otherwise ends up out of action, they're the number one candidate for a Legacy Character. (if it sticks)
Particularly common sidekick types include:
- Kid Sidekick
- Damsel in Distress
- Cowardly Sidekick
- Bumbling Sidekick
- Hypercompetent Sidekick
- Chessmaster Sidekick
- Non-Human Sidekick
- Fairy Companion
- Psycho Sidekick
- Older Sidekick
- Reckless Sidekick
- Talking Appliance Sidekick
- The Watson
- Bastard Understudy is an evil variant.
- Non-Player Companion is a gameplay implementation of this trope.