The few women in The Merchant of Venice are generally portrayed as strong characters but within the restrictions of 17th century Elizabethan England.
Portia has the strength of character to remain loyal to her late father as she is honouring his wishes by marrying according to whomever chooses the correct casket (Act1.ii)This shows that she plays by the rules. The Law (and thus the 'rules')is very important in "The Merchant of Venice"
She dresses as a man, thereby again keeping within the constraint of the Law but uses her strong will to manipulate situations. Even though she is a woman in an obviously man's world, she is not phased by dealing with Shylock and is quite masterful in ensuring the outcome of his trial. (Act IV)
Portia does also retain typically female characteristics and is seen as compassionate although wily.
Jessica is seen as strong-willed in her defiance of her father (Shylock) but she is led by a need to be conflicting - like a naughty child who will disobey simply to upset the parent. She sells the ring in a frivolous act. This indicates that although she appears strong, she lacks strength of character and mostly reacts to circumstances rather than manipulating them.
Nerissa, Portia's servant is typical of the era and although she shows no real strength of character and conforms to what is expected of her - her marriage mimics Portia's - a very accepted norm at that time, her very acceptance of her circumstances in itself shows depth but it is open to interpretation.