Sidekicks serve some very useful functions in constructing a story. a sidekick can provide comic relief, prod a reluctant hero along the journey, and serve as an important element of world-building.
Think about Anakin Skywalker lovingly building C3PO from scrap metal because he didn't have any friends. This shows the boy's genius, and also his capacity for love. As a sidekick to Batman, the many Robins have been privy to all the secrets of Wayne Manor. And we get to see the cold Dark Knight in the role of a father. Samwise Gamgee kept reminding Frodo of his humanity (and second breakfast) in Lord of the Rings. The Lone Gunman showed us that Mulder and Scully were not the most tragic nerds on the block. And we can well imagine that Kitt hauled Michael Knight home safely after too many bullets, babes, or beers.
What about you? Do you have a favorite sidekick from a favorite book, movie, or television program? Why?