Main Characters:
* Sherlock Holmes: The brilliant detective, renowned for his deductive reasoning and observational skills.
* Dr. John H. Watson: Holmes' loyal friend and chronicler, often serving as the reader's point of view.
* Mary Morstan: A young woman seeking information about her father's disappearance and the mysterious "Sign of Four" she receives.
* Jonathan Small: The villainous, but ultimately tragic, mastermind behind the theft of the Agra treasure and the "Sign of Four."
* Thaddeus Sholto: Jonathan Small's former partner in crime, who inherits a portion of the Agra treasure. He is murdered before the story begins.
* Athelney Jones: A Scotland Yard detective, often ridiculed by Holmes for his overly methodical and less intuitive approach.
Other Important Characters:
* Mrs. Hudson: The landlady of 221B Baker Street, where Holmes and Watson reside.
* The Tonga Tribe: A group of natives from the Andaman Islands, loyal to Jonathan Small and used to carry out his plans.
* The "Sign of Four": The name given to four unique objects: a silver pin, a small, decorated box, a piece of lead pipe, and a dog's collar. These items are associated with the theft of the Agra treasure.
Supporting Characters:
* Bartholomew Sholto: Thaddeus's brother, who inherits a portion of the Agra treasure and becomes a target for Jonathan Small.
* Mrs. Sholto: Thaddeus and Bartholomew's mother, who is concerned about their safety.
* Ms. Morstan (Mary's mother): A woman who was once in love with Thaddeus Sholto and who died under mysterious circumstances.
* Inspector Bradstreet: A Scotland Yard inspector who works with Athelney Jones.
It's important to remember that while the story primarily focuses on the main characters, many supporting characters add layers to the plot and contribute to the overall atmosphere of the novel.