Bosola is the spy planted by Duke Ferdinand as the stable master at the Duchess’ estate. He is a man who is used to doing the dirty work for others: before the events of the play he spent time in jail for murdering a man on the orders of the Cardinal. He is also aware that the men who do the dirty work seldom actually get the rewards promised them, as the Cardinal refuses to speak or be seen with him. Even so, though he feels guilty for all of his actions – and does not even want to become a spy when Duke Ferdinand offers him the payment to become one – he feels that it is his duty to obey the Duke and accepts that to follow orders he must become corrupt.

After he participates in the torture of the Duchess, though, his guilt becomes so great, and Ferdinand’s refusal to pay him for his services so outrageous, that he switches sides and plans to help Antonio and kill both Ferdinand and the Cardinal.His plans go awry and he accidentally kills Antonio. Suggesting, that it is not so simple to suddenly become good and moral, but he does willingly sacrifice himself and badly wound the Cardinal and kill Ferdinand.

The Duchess, a young widow and the ruler of the Italian town of Amalfi, is the intelligent, kind, virtuous sister of the Cardinal and the twin of Duke Ferdinand. Her brothers have prohibited her from remarrying because, they argue, her remarriage would ruin her honor and the honor of the family. The Duchess also seems to understand that her brothers have more nefarious aims, such as ensuring their own chances at inheriting her fortune, though her understanding is implied mainly by her actions rather than her words. Independent and defiant of her brothers’ wishes, the Duchess decides to secretly marry her steward, Antonio, for love, and has three children with him. She keeps both the marriage and the children secret because she understands the threat her brothers would pose to her family should they find out. And, in fact, once her brothers do find out the Duchess seems almost completely unable to protect herself or her children. Perhaps because she is a woman, she lacks her brother’s political power, and they quickly banish and split up her family. They then imprison, torture, and strangle both her and her children. Through all of these trials, the Duchess remains virtuous and good, and she faces both torture and death with bravery and dignity.

The Cardinal is the brother of Duke Ferdinand and the Duchess. Though he is a religious figure, he is in fact just as immoral and despicable as his brother, facts made clear by his attempt to bribe his way into being pope, the fact that Bosola once killed a man on his orders, and the affair he carries on with Julia, Castruccio’s wife. Like Ferdinand, he tries to prevent the Duchess from remarrying in order to preserve his sense of his family’s purity and honor as well as his access to the Duchess’s wealth. Unlike the wild Ferdinand, though, the Cardinal is careful, calculating, and controlled: he refuses to interact personally with the spy Bosola, and he threatens to walk away when Ferdinand becomes too overt about his plans for revenge on the Duchess. While it’s never explained whether the Cardinal is upset by Ferdinand’s violence or just trying to shut Ferdinand up in order to keep themselves looking clean while they plan their revenge, the fact that the Cardinal is entirely capable of murder – he later poisons Julia, after all, when she learns his secrets – suggests that it is the latter. Though he is aware of the religious consequences of his actions, he wields religion only as a tool to maintain his power. He never seems to feel true guilt for his actions, and there is a sense of poetic justice in the fact that ultimately the Cardinal dies after being stabbed by Bosola, the spy he used but refused to engage with or even pay, and his own brother, Ferdinand, who by the end of the play is guilt ridden and insane.

Antonio is the Duchess’s steward, and very capably runs the Duchess’s estate. Despite the fact that he is neither wealthy nor high-born, the Duchess considers him to be a “complete” man, and the two of them secretly marry. He clearly reveres the Duchess – he is marrying for love, not just money. He is also knowledgeable about people: even early in the play he knows that Ferdinand and the Cardinal are duplicitous and murderous. Despite his knowledge of their characters, though, he proves entirely incapable of protecting his family from Ferdinand or the Cardinal. And while that failure seems to stem from his lower-class status and lack of political power of any sort, and while Antonio never seems anything less than morally good, his plan to sneak into the Cardinals home at the end of the play in order to try to convince the Cardinal to make amends also seems incredible naïve. During this effort, he is accidentally killed by Bosola, who mistakes Antonio for someone else.
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