Blood stems from the liver and correlates with the spring season and the attributes of hot and moist, making air its element. An individual who is innate sanguine character is full of life, robust and full-bodied. Yellow bile arises from the gall bladder and is linked with the summer season. Its attributes are hot and dry and its element is fire. A person with a choleric temperament is naturally considered angry and hot-tempered. Black bile originates from the spleen and is associated with the autumn season. Its attributes are cold and dry, making its element earth. A Melancholic individual can be depressed, moody and cynical. .

             Webster has modeled each of his four main characters after one of the humors, using the theory and its explanations as a basis for their characterization and an explanation for their actions. Unlike the explanation given by the medical community at that time, Webster used a bit of artistic license and implied that the cause of any imbalance of humors and subsequently the cause of mental illness is a result of immoral life choices made by the individual. The duchess is humble, noble and kind and the most mentally stable of the four main characters. Ferdinand is her exact opposite; he is sinful, vengeful, angry and ill tempered and throughout the play unravels to become the most unhinged of the four.

             The duchess is clearly the protagonist of the story (note the title of the play) and is the only one of the four main characters not adversely affected by her humor. In the play she is characterized as a morally sound noble individual through action as well as impression: "But for their sister, the right noble duchess:/ You never fixed your eye on three fair medals/ Cast in one figure, of so different temper (1.1.187-89). Antonio, who serves as a sort of moral compass during the beginning of the play, offers the above quoted opinion of the duchess. Antonia's description of each character is proven correct throughout the play by their actions and behaviors.