The Duke is introduced in the same manner as the Duchess: by a portrait in their abandoned villa.
“It was a narrow-browed face, sallow as a wax effigy, high-nosed and cautious-lidded, as though modeled by priestly hands; the lips weak and vain rather than cruel; a quibbling mouth that would have snapped at verbal errors like a lizard catching flies, but had never learned the shape of a round yes or no. One of the Duke’s hands rested on the head of a dwarf, a simian creature with pearl earrings and fantastic dress; the other turned pages of a folio propped on a skull.”[1]
The image captured by the priest paints a certain picture in the reader's mind: The Duke is presented as a man who seems pious and, though well-educated, potentially petty-minded and indecisive with an at best indifferent or at worst cruel streak. Further along, the reader learns that the Duke is also a more quiet person. This immediately puts him at odds with his wife, the vivacious and high-spirited polar opposite, nicely illustrated in the following paragraph:
“The Duke was a silent man, stepping quietly, with his eyes down, as though he’d just come from confession; when the Duchess’s lapdog yapped at his heels he danced like a man in a swarm of hornets; when the Duchess laughed he winced as if you’d drawn a diamond across a window-pane. And the Duchess was always laughing.”[2]
He is absent from the villa at Vicenza more often than not (and one could conjecture whether or not he potentially was having an affair of his own) and his presence does not seem to be a source of joy; the opposite seems to be the case, with the Duchess being especially happy when he is not present.[3] His love and affection for the Duchess does not make itself known in the short story. No instances of him showing physical affection or emotional attachment are relayed by the old man. Whether or not he loved his wife truly is unclear, though in the latter half of the story an interesting trait comes into play: his possessiveness. The narrative strongly implies that the chaplain, scorned by the Duchess, takes to spying on her and Ascanio, and feeds the Duke information about her adulterous behavior. This must spark a strong reaction within Duke Ercole. Interestingly, he holds off his anger, neither accusing her directly or turning to physical violence. In step with his intellectual prowess, he goes about matters in a cool and collected way. He orders a Bernini sculpture targeted to both let his wife feel the burn of his scorn and to, in a sense, replace her vital, sexually active and out-of-his-control physicality with the hard revenge of sculpted marble. To add insult to injury, he orders the statue placed over the supposed meeting place. Exactly where, for the Duchess, the sacred meets the sexual, and romantic fulfillment meets attained agency, the Duke very clearly robs her of all of these. Although he has 'won', it is not sufficient to leave it as a figurative chastisement. The ultimate punishment for her bid for an unfettered and more fulfilled life is death, enacted in the same cautious and planned manner as the death of her lover. Violante is robbed of her freedom as Ascanio is robbed of his life. Having enacted his will and exercised his revenge, the Duke's final satisfaction is not only her spiritual but also her physical death.
After this entire ordeal, he tells his family the Duchess has passed away after what could be called food poisoning and takes a new wife who bears him a son and five daughters[4]. If he has indeed murdered the Duchess, he has gotten away with it unpunished and unscathed. He even finds what could be seen as a happy marriage, for it is certainly more fruitful than this union with Violante, and perhaps also a happier life (though that is pure speculation). What could be seen as the villain of the story - at least from the Duchess's perspective and from that of those that empathize with her situation - has walked away triumphant, showing once again that one cannot rely on Wharton for a happy end in the same way one equally cannot rely on the hand of fate to paint a happy and just end in life outside of fiction.
[1] Wharton 1901, p. 3
[2] Wharton 1901, p. 8
[3] Wharton 1901, p. 12
[4] see Wharton 1901, p. 18