Dr. Watson's character in The Hound of the Baskervilles is no different from his character in all the other Sherlock Holmes stories. He is a conservative, unimaginative Britisher who is devoted to Sherlock Holmes and acts as his biographer. He enjoys sharing in Holmes's adventures and frequently acts as his assistant. Watson is a foil to Holmes because he is not brilliant or imaginative; he is orthodox in his behavior and opinions, whereas Holmes is a complete individualist. Both men, however, are patriotic Englishmen and politically conservative.
In The Hound of the Baskervilles, Sherlock Holmes stays out of the picture for most of the book. I believe this story was converted into a Sherlock Holmes story and had not originally been intended as such. That would explain why Holmes is absent so much of the time and why Watson is so much on his own.
Watson's outstanding traits are loyalty, courage, and kindness. He is a doctor, which means that he sympathizes with people who have troubles. Holmes, by contrast, is not especially interested in people but in the intellectual problems and challenges their troubles present. Watson's greatest weakness is that he is not intelligent. He would like to be a detective like his friend Holmes, but he has no imagination and cannot make logical deductions. You could describe Watson by contrasting him with Holmes, as this was Doyle's intention.
Arthur Conan Doyle got the idea for writing Sherlock Holmes stories from Edgar Allan Poe's two or three "tales of ratiocination," especially from Poe's "The Murders in the Rue Morgue." These stories are told by a narrator who, like Watson, is a friend of the amateur detective-hero and who accompanies him on his investigations but cannot see what the hero sees. The other important Poe story is "The Purloined Letter."