We see the same friendship between Agatha Christie's Hercule Poirot and his most frequent assistant, Captain Arthur Hastings. Like Watson, Hastings serves the role of biographer. However, he does much more than that. He serves as a sounding-board for Poirot's ideas, and Poirot recognizes that. In Thirteen at Dinner (AKA Lord Edgeware Dies), Poirot tells Hastings how much he values Hastings' perspective (although admittedly, Hastings doesn't at first recognize Poirot's words for the compliment they are). In The ABC Murders, Hastings calls attention to an important clue that Poirot nearly misses, and Poirot thanks him for preventing Poirot from committing the "unforgiveable sin" of "overlooking the obvious."
In more modern crime fiction, the sidekick has taken on an even more interesting and important set of roles. For example, in Janet Evanovich's Stephanie Plum series, Stephanie is often assisted by Ricardo Carlos Manoso, AKA Ranger. Ranger is in top physical form and goes with Stepahnie on many of her fugitive apprehension cases. He's saved her life more than once. Ranger, though, is an interesting character in his own right. He's a former member of the Special Forces, and has a colorful, if somewhat mysterious, past.
One of the most fascinating sidekicks I've read about is Mma Grace Matsuki, Assistant Detective to Mma Precious Ramotswe in Alexander McCall Smith's No. 1 Ladies' Detective Agency. Mma Matsuki is a highly competent, efficient secretary who scored an unheard-of 97% on her exam at the Botswana College of Secretarial and Office Skills. She is much more than a secretary, though. She's highly intelligent and has deductive powers of her own, so when Mma Ramotswe recognizes her skills, she promotes Mma Matuski to the rank of Assistant Detective. She also becomes Assistant Manager of Tlokweng Road Speedy Motors, and shows her organizational skills as she manages both enterprises and investigates her own cases. She also opens and runs the Kalahari Typing School for Men, takes care of her ailing brother and never forgets to send money back home to her family. Far from a mere biographer, Grace Matsuki is a strong central character in her own right.
Who are your favorite sidekicks? What do you think sidekicks add to a good mystery?