Holmes is an eccentric consulting detective operating out of his Baker Street apartment. He approaches mysteries with a detached, scientific mindset, preferring everyday cases that challenge his observational skills. As a self-made investigator operating outside of traditional law enforcement, he values intellectual stimulation and logic over sentimentality.

Dr. Watson is a military veteran who recently moved out of the Baker Street apartment after marrying. He maintains a close friendship with Holmes, frequently returning to visit and assist with cases out of a desire for action. He serves as a grounded, conventional contrast to his friend's bohemian lifestyle.

James Windibank is Mary Sutherland's controlling stepfather who married into her family and immediately took charge of their finances. He strongly opposes Mary's decision to consult a detective about her missing fiancé. He frequently travels to France for business, leaving his wife and stepdaughter in London.

Mary is a shortsighted young woman who works as a typist to support herself without burdening her family. She lives under the strict control of her stepfather, who manages her inheritance and disapproves of her seeking outside help. Despite this opposition, she earnestly pursues answers regarding her recently vanished fiancé.

Mary Morstan is Dr. Watson's wife. Her marriage to Watson provides him with a conventional domestic life that contrasts sharply with the bizarre cases he pursues with Holmes.