Julia is presented by Webster as very much the foil to the Duchess. Whereas the Duchess is given to us as a respectable aristocratic lady of the utmost refinement and constancy, Julia is portrayed as a scarlet woman, brazen in expressing her overpowering sexuality. She's an inveterate pleasure-seeker, flitting back and forth between one man and another whenever the mood suits her.

It's clear from Julia's unpleasant demise—murdered by her lover, the Cardinal, using a poisoned Bible, no less (how's that for symbolism?)—that Webster does not regard her behavior as in any way appropriate. The prevailing moral standards of the time dictated that women, especially those of noble birth, should maintain chasteness and purity at all costs. Julia, in departing from those standards (and that's putting it mildly) has suffered what most people at that time would've considered a richly-deserved fate.

In stark contrast, the Duchess's fate is truly tragic because, unlike Julia, she unfailingly lives up to the ideal of aristocratic womanhood. Despite the constant taunts and accusations from her brothers concerning her clandestine marriage to Antonio, she can hold her head up high as a virtuous lady of quality. Though possessed of a powerful sexuality, the Duchess chooses to express it within the confines of marriage, albeit a marriage with a man some way beneath her on the social ladder.

It might be possible to see Webster exploring the power of female agency in both the Duchess and Julia. The play celebrates the Duchess's ability to set her own agenda and pursue it without regard for social expectations or the dictates of her brothers. The Duchess is warm and noble, with a fully admirable expression of femininity. She seems entirely charming, intelligent, and virtuous, willing to bestow her love and privilege where she finds honor. Julia is more calculating and less morally admirable, yet she too is using her opportunity to advance her own interests.

In both cases, Webster does not afford a "happy ending" or any sense of escape for women who do pursue their own interests. Of course, he doesn't offer this to male characters either. In both cases, Webster's tragic vision suggests that forces close in upon one, whether one is virtuous or villainous. The Duchess's death feels more tragic but no more nor less avoidable than Julia's.

Some might argue that Webster is creating a dichotomy with the two women as the Duchess is noble, keeps her feelings to herself, and plays the roles that a woman should by being proper and following the will of the men around her for the most part.  Julia, on the other hand, acts independently and tries to use her power as a woman to seduce Bosola and eventually pays for her dishonesty by kissing the poisoned book the Cardinal offers her.

In portraying the Duchess as the wonderful woman who is beloved by all around her for being so honest and virtuous and then juxtaposing that woman with Julia who is reviled and eventually killed because of her dishonesty and far more independent and devious nature, Webster does create a commentary about how a woman ought to behave with clear guidelines and rewards for the one who does behave properly.

Yet there are things about both women that suggest that Webster is not against a woman being independent.  The fact that the Duchess chooses to marry someone that is lower than her social and political standing suggests that she is not entirely bound by custom.  This is mirrored by the willingness of Julia to use her powers of persuasion and her sexuality to try and get what she wants suggesting that some level of independence is acceptable and perhaps even celebrated.