A character sketch is a short written description of a person's qualities. Such a description will, therefore, include details of the person's personality as well as information about his or her physical attributes.
We learn much about Portia in act one. When Bassanio describes her to Antonio in scene one, we learn that she is a wealthy heiress, "richly left," that she is beautiful ("she is fair, and, fairer than that word"), and that she possesses many other amazingly benevolent qualities (she is "Of wondrous virtues").
In Bassanio's contention, then, Portia would be an excellent bride. This fact is proven when he speaks about how men of stature, wealth, and prestige have come from far and wide to woo her, as he says, "the four winds blow in from every coast / Renowned suitors."
It is also evident that Portia is blond and that her beauty has driven many on a quest to win her affections. Bassanio states that "her sunny locks / Hang on her temples like a golden fleece."
Bassanio alludes to Jason, an ancient Greek mythological hero who went on a quest for the golden fleece, when he mentions how determined Portia's suitors are to obtain her love.
Portia also possesses mature self-knowledge. When she speaks to her handmaiden, Nerissa, she tells her that she believes herself to be better at teaching than she is at following her own advice. This suggests that she is stubborn. We learn, however, that in spite of her concern about the unfair demands of her father's will, she respects his wishes and will follow them. This means that she is loyal and dutiful.
Portia is also fastidious and seeks only the best for herself. She has, thus far, carefully scrutinized her suitors and seems displeased with all of them. None of them have met her high standards, and she wishes that all of them would just leave. There is also a hint of dishonesty when she suggests that the Duke of Saxony's nephew could be misled into choosing the wrong casket by placing a glass of wine "on the contrary casket."
Portia also displays prejudice when she informs Nerissa that if one of her suitors "have the condition of a saint and the complexion of a devil, I had rather he should shrive me than wive me." This suggests that she would rather have such a person listen to her confess than become his wife.
Portia is apparently not perfect, but despite her shortcomings, she would be, in the eyes of her admirers, an ideal partner.