In the same scene, she showed what a good judge of character she was, when discussing the various suitors with Nerissa. She did not break her word, but managed to get the man she wanted by hanging on to her belief in love, and then offering him the duty and love she once gave her father.
Next, is her quick mind and intelligence; she saw right to the crux of any problems and thought of solutions quickly. When Bassanio got the letter about Antonio's problems, she offered the money right away to get them out of trouble. It was a rational and decisive solution. When the offer of money did not work, Portia looked for another way out and sought expert help to put her plan into practice.
Her intelligence meant she had a realistic view, and helped her recognize that a rich young woman would have no influence or power, but a clever young lawyer would. It also made her aware that expert advice was necessary, as was the quick decision to get that advice. Her great reasoning and negotiating powers allowed her to understand that Shylock wanted to use the law, so she would use it too, to get a good outcome for all.
The third and most important characteristic is her ethical and fair way of dealing with everything.