Mary Morstan is a fictional character from the Sherlock Holmes stories written by Arthur Conan Doyle. She first appears in "The Sign of the Four," where she seeks the help of Sherlock Holmes and Dr. John Watson to uncover the mystery surrounding her missing father and a valuable treasure. Mary is portrayed as intelligent, resourceful, and strong-willed. Throughout the story, she develops a close relationship with Dr. Watson, ultimately becoming his wife. Her character adds depth to the narrative and highlights the themes of loyalty and love in the face of adversity.