The most important thing to know about Isabella is that she's a virgin and she plans on staying that way. Forever.
When we first meet this young woman, she's getting ready to join a convent that has some very, very strict rules for its nuns. If, for example, a sister wants to have a conversation with a man, she can't show him her face at the same time and she has to have this conversation in the presence of the prioress (head nun). Isabella's OK with this. In fact, she says she wishes the convent were even more hardcore in its rules and regulations than it already is!
And have you nuns no farther privileges?
Are not these large enough?
Yes, truly. I speak not as desiring more,
But rather wishing a more strict restraint
Upon the sisterhood, the votarists of Saint Clare. (1.4.1-5)
OK. We're not judging. There's nothing wrong with wanting be a nun, but we wonder why Isabella wants to lead such an extreme lifestyle. Is it simply because our girl wants a closer relationship with God, or is there more to it?
When we think about how Vienna has been overrun by illegal brothels and high rates of sexual deviance, it seems like Isabella might be seeking refuge in the convent from the corruption and moral decay that's associated with a character who is Isabella's polar opposite, Mistress Overdone.
We also know a thing or two about Isabella's attitude toward sex. When Isabella learns that her brother has been sentenced to death for "fornicating" with a woman he's not married to, Isabella is horrified by her brother's sinful behavior. Nevertheless, she visits Angelo and begs for her brother's life (after acknowledging that Claudio should be subject to some kind of punishment, of course). Still, when Angelo propositions Isabella, he gives our girl a horrible ultimatum: she can have sex with Angelo and save her brother's life, or she can remain a virgin and watch her brother die.
Isabella, who cannot even bring herself to say the word "sex" out loud, refuses and declares "Then, Isabel, live chaste, and, brother, die. / More than our brother is our chastity" (2.4.198-199). Isabella has clearly decided that her chastity is more valuable than her brother's life.
Interestingly enough, in Shakespeare's main source for the play (Promos and Cassandra), the young woman who corresponds to Isabella's character actually sleeps with a corrupt judge to save her brother's life. Why does Shakespeare deviate from his source? Is the play asking us to decide whether or not Isabella makes the right choice? If so, why does Shakespeare let Isabella off the hook by introducing the Duke's bed trick?
For some literary critics, Isabella comes off as an uptight ice-maiden. For others, Isabella is a sympathetic victim who is placed in an impossible position. How do you view her?
To complicate matters, Shakespeare introduces another ambiguous matter at the play's end. When the Duke (out of nowhere) proposes to the apprentice nun, Isabella remains completely silent (5.1). Is Isabella speechless because she's so happy that she's engaged? (If so, when did Isabella decide she no longer wants to be a nun?)
Or, is Isabella stunned into silence because she's the recipient of yet another unwanted proposition? (If this is the case, then Shakespeare's attitude toward marriage and gender dynamics of power is pretty cynical, don't you think?)