Julia is the aging courtier Castruccio's wife and the Cardinal's mistress. Her character is more or less one-dimensional. It is summed up at the end when Bosola calls her the "lustful" Julia.
Julia likes men, and when we first meet her, the Cardinal is annoyed that her elderly husband is returning to court. The Cardinal tells her she should be grateful to him for offering her the sex he says her husband can't give her. He says Castruccio keeps her like an "elephant," well fed and kissed, but that he can only tantalize her by strumming the "lute" of her body without being able to play it, an allusion to the elderly man's impotence.
Delio comes in and offers her gold to resume his affair with her, while, later, when Julia sees Bosola, she is attracted to his "form." She boldly and quickly approaches him with the idea of an affair and tells him that if he hides in her closet the next day, he will overhear secrets when the Cardinal comes.
Julia, however, is over her head in trying to double-cross someone as ruthless as the Cardinal. He reveals to her his role in having the duchess murdered, only because he has already decided to kill her, which he does by having her kiss a poisoned Bible.
Julia essentially has two roles in the play: In the first, she is a foil or opposite to the sexually constant, faithful duchess, who represents pure womanhood. Julia, in contrast, is a symbol of the false and corrupt court, willing to betray her husband, lie, and offer sexual favors for her own pleasure and gain. Webster is not overly condemnatory of her: after all, her behavior pales in contrast to that of Ferdinand and the Cardinal.
Second, she helps to illustrate the theme of reciprocity located in the term "gratitude." As far as the duke and Cardinal are concerned, everyone of a lower class is supposed to have gratitude towards them, but they feel no compulsion to return the favor, a fatal flaw of arrogance that becomes their downfall in a world that has become wholly transactional.