Who are the main characters in The Hound of the Baskervilles?

The main character in the book, of course, is the famous detective Sherlock Holmes.  Holmes is a brilliant thinker and delights in the excitement of "the chase".  In addition, he is rather an eccentric character who takes only those cases which interest him, has a penchant for dramatic flair, and is capable of forming amazing deductions from seemingly insignificant clues.

Dr. John Watson is Holmes's friend and assistant.  He provides a good balance for the detective's quirky personality, being himself intelligent, energetic, courageous, and loyal.  It is Watson who narrates the story, as he does almost all of Sherlock Holmes's adventures.

Sir Henry Baskerville, the last known heir in the Baskerville line, has a fiery temper and a mind of his own.  Although he is an impeccable gentleman, he is independent and tends to follow  his own inclinations, sometimes making it difficult for Holmes to both ensure his safely and solve the case.

Dr. James Mortimer is a young country physician and a friend of Sir Henry.  It is he who first appeals to Sherlock Holmes for help in keeping the heir of the Baskerville family safe.

The villain in the story is Stapleton, a respected naturalist and a reknowned authority in entymology.  Despite his innocuous appearance he has a keen mind, and has devised an ingenious way to use the legend of the Hound of the Baskervilles to his advantage.

Who are the credible characters in The Hound of the Baskervilles?

As with most crime fiction, most of the characters withhold information or lie about it. What creates and resolves suspense in this sort of story is precisely the way readers are only gradually given the information needed to fully understand the events we witness. For characters:

Narrator: The third person narrator is reliable but is mainly restricted to the viewpoint of Watson and only knows or reveals information to us gradually.

Watson: He has an open and honest character and tells the truth, but is not as intellectually acute as Holmes, and does not always understand what lies behind the events he observes and records.

Holmes: The consulting detective is brilliant and seeks and discovers the truth in all of the works featuring him. At times, he will lie or engage in other forms of deception to achieve his ultimate ends, but in the end he will reveal the truth.

Sir Henry Baskerville and Dr. Mortimer: Both honest but puzzled by events.

Barrymore and Mrs. Barrymore: Lie initially to help Selden but otherwise truthful. 

Mr. Stapleton: Not credible.