Watson is a foil, or contrast, to Holmes throughout the novella. Watson's own inability to puzzle out rationally what is going on with the mystery highlights the power and logic of Holmes's superior intellect. As Holmes says to Watson:
"How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?"
Watson also acts as an emotional foil to Holmes, who remains wedded to the rational throughout. Watson, in contrast, falls in love with Mary Morstan, the woman who comes to Holmes hoping he will help her solve the mystery of what happened to her missing father, as well to help her discover the meaning of the pearl she receives every year. While Holmes focuses his laser sharp mind on analyzing what has happened, Watson deals with a dizzying array of emotions about Mary. Although he is in love with her, he hesitates to declare himself when he thinks Mary is the heiress to a fortune, for fear she will think he is after her for the money. But when he finds out she will not inherit the bulk of the Agra treasure, he proposes to her. Holmes, meanwhile, stays aloof from love entanglements.