Portia is one of Shakespeare's heroines who turns out to be the hero of the play. Dressed in men's clothes and playing the part of a young lawyer in the courtroom where Antonio's life hangs in the balance, Portia gives an heroic speech that saves the day.
Portia's primary qualities are a love of delicacy, goodness, compassion and mercy. As suitor after suitor comes to try for her hand according to the guidelines in her father's strange will, she mercifully finds ways of dismissing them with the truth, but the truth compassionately disguised so that it doesn't have a painful knife's edge to it. She doesn't tell them she dislikes them, she tells them that they have as fair a chance at winning her hand as any other she has seen.
In private, her great reasoning power and witty facility with words prepares us for the role she later plays as a lawyer. She must have wit and wisdom to win in a court of law and this is precisely what she does have. She elaborates for Nerissa and us on her disdain for the imperfections of the suitors, a disdain that she delicately hid from their knowledge.
Finally, it is Portia's plea for mercy on Antonio's behalf that saves him from Shylock's exacting knife of justice and revenge. It is mercy that we have seen at work as Portia turns away unwanted suitors. In both cases, Portia with her suitors and Shylock's court case, the facts and the truth are clearly known. In court, Shylock presents the facts; at her estate, Portia presents the facts. Portia's wisdom sets the precedent for Shylock to extend mercy regardless of the truth of the facts.