Bosola is a somewhat Machiavellian character, used by Duke Ferdinand to spy on the Duchess of Malfi. Though ostensibly her stable manager, he is there to keep tabs on her movements at the behest of his royal patron.

This isn't the first time that Bosola has been involved in such secret, clandestine business. Back in the day, he carried out the dirty work of another high-placed personage, murdering a man at the behest of the Cardinal, Duke Ferdinand's brother. After what Bosola did for his brother, the Duke clearly felt that Bosola was just the man to spy on the Duchess.

Yet Bosola proves not to be a willing lackey. He actually develops moral qualms about doing the dirty work for evil aristocrats. If he obeys the Duke, it is only out of a sense of loyalty; he is not in it for the money.

After participating in the torture of the Duchess, Bosola's guilt is such that he can no longer work for Duke Ferdinand. Thus, Bosola switches sides and helps Antonio kill the Duke and the Cardinal. In the event, he accidentally kills Antonio, but he nonetheless manages to do some good in his final act by also killing Ferdinand and fatally wounding the Cardinal.

Bosola is a hard character to classify within the narrative of The Duchess of Malfi. His worldly role is that of a spy and assassin. On a literary level, he is technically an antagonist until his employers skip one payment too many. The Duchess's brothers hire him to spy on their widowed sister in her household once they grow suspicious that she might marry against their wishes, thus preventing them from taking control of her estate should she bear children. However, matters do not go as planned.

Bosola is complex because he does not want to be a villain. He hates the idea of spying on the Duchess and even comes to admire her for her goodness, but he also wants the money promised him. Being a former galley slave, he feels he has little choice but to do the dirty work of others. Once he is forced to help torture and kill the Duchess on her brother Ferdinand's orders, Ferdinand does not pay him for the work. At that point, Bosola turns on his former employers, comforting the Duchess in her last moments and then allying himself with Antonio. Unfortunately, he accidentally kills Antonio, but he manages to eliminate the Duchess's brothers, too, before he is stabbed to death himself.

Bosola is the only character in the play who truly changes. He goes from being a killer motivated by money and security to a man seeking redemption by helping the good people he wounded most. This makes Bosola especially unique since he is technically the main avenger in this most atypical of revenge tragedies. Unlike an avenger such as Shakespeare's Hamlet, Bosola is also one of the initial perpetrators, which adds a redemptive edge to his role in the story. While Bosola's redemption does not go as planned, he at least succeeds in bringing all of the Duchess's killers to justice, including, ironically, himself.

Bosola is a complicated and fascinating character. He acts the role of both villain and avenger, working first against and then for the duchess.

First, Bosola agrees to work as a spy and also murder the Duchess for Ferdinand and the Cardinal, even though he knows they are evil and their court corrupt. He realizes he has entered into a wicked bargain. Ferdinand offers him a respectable job, something he covets very highly, as an ex-convict who spent seven years as a galley slave because of the murder he committed earlier. As Bosola puts it:

For the good deed you have done me, I must do
All the ill man can invent!

Later, however, the cynical Bosola grows to admire the Duchess for her genuine goodness. He sees the kind works of her "white hand." He wishes he worked for her, but he has made his bargain. He kills her, as is expected. However, when Ferdinand reneges on paying Bosola, Bosola gets angry. He says,

I served your tyranny, and rather strove
To satisfy yourself than all the world;
And, though I loathed the evil, yet I loved
You that did counsel it, and rather sought
To appear a true servant than an honest man . . .

Killing the Duchess, Bosola says, was "much 'gainst mine own good nature." He decides to avenge the Duchess by killing her brothers, though he is killed in return.

In a purely literal sense Bosola's role is that of the hired assassin. More fundamentally, he represents the repentant sinner by the end of the play; he moves from performing cold-blooded acts of murder, simply for money, to commentating on violence and evil. He does not turn good altogether and dies in misery and confusion, but his move towards repentance throws into sharp relief the greater villainy of the  the Cardinal and Ferdinand - particularly the former.