He is Sherlock Holmes’ sidekick and

He is Sherlock Holmes’ sidekick and the narrator of the story. He is a retired military doctor. He is a true friend of Holmes and puts a great deal of faith in him. He doesn’t always pick up on everything, but he does help Holmes out. He is the one that investigates while Holmes is busy.
In Hound, Watson tries his hand at Holmes' game, expressing his eagerness to please and impress the master by solving such a baffling case. As sidekick and apprentice to Holmes, Watson acts as a foil for Holmes' genius and as a stand-in for us, the awestruck audience.