If you are analyzing Portia's character, then here are several attributes to discuss based on the trial scene.
1. Portia is clever and self-assured: She thinks nothing of entering the male world of justice and business. While she must disguise herself as a man to participate in the trial, rather than view that as an insult, Portia finds humor in it because she knows that she is outwitting the men at their own game. Similarly, while many of the Venetians seem to fear Shylock, Portia toys with him and lectures him on mercy.
2. Portia is either compassionate and ironic: Her "Quality of Mercy" speech which appears in the trial scene demonstrates a willingness on her part to provide Shylock with an out if he chooses to take it. Going into the trial, Portia knows that she has the upper hand, but rather than immediately identifying the egregious flaw in Shylock's contract, she offers him an opportunity to show mercy just as he would want it shown to him. In addition to illustrating Portia's compassion, the trial scene (especially the "Mercy" speech) exemplifies Portia's ironic nature. She warns Shylock about the irony for insisting on the letter of the law--that eventually someone will insist on the letter of the law when dealing with him--and in doing so, presents a lesson to the whole court about the irony of their justice system.