Daniel de Bosola acts as a spy for the duchess's brothers in the duchess's own household. He himself holds no ill will against the duchess and even comes to admire her integrity and courage. He is also distrustful of both Duke Ferdinand and the cardinal, knowing that such corrupt men rarely uphold their promises when they offer a reward for dirty work. Still, he agrees to spy on the duchess, reporting back any behavior that might suggest that she has defied her family's wishes against marriage or sexual activity in general. He knows this is wrong, but he feels it is his duty to do such work.

Bosola soon finds out that the duchess is pregnant. He gives her apricots to test this theory (at the time of the play, it was believed that apricots could induce labor), and the experiment proves successful. Bosola's reports are what lead to the duchess's downfall, since her brothers take swift action to punish her for going against their wishes (and, really, their economic interests, since if she lived childless, they would inherit her property).

Torturing the duchess and being complicit in her murder is what breaks Bosola. He turns on his employers, killing Ferdinand and injuring the cardinal before dying himself.