The Prince of Morocco is a proud, valiant man. However, because he is foreign and has very dark skin, he displays some anxiety about Portia's acceptance of him.
The Prince's focus upon his dark skin and Portia's impression of his physical appearance indicates that he values the superficial. As proof that he is a true prince, he offers to cut his flesh and show her that his blood is redder than any man who is fairer than he. He says he would change nothing about himself but to change her opinion of him:
...I would not change this hue,
Except to steal your thoughts, my gentle queen. (2.1.11-12)
He also believes that he deserves Portia:
I do in birth deserve her, and in fortunes,
In graces, and in qualities of breeding.
But more than these, in love I do deserve. (2.7.34-36)
Then, too, he may possess a secret fear that Portia does not desire him. Also, this focus upon the superficial extends itself to his choice of the caskets as he misjudges which one is the correct choice because of the appearance the caskets. For, he cannot believe that Portia's father would put her portrait in anything but the gold casket since her beauty is deserving of nothing but the best.
When he does not make the correct choice, he says that he leaves in despair, but he will not linger.
Then, farewell, heat, and welcome, frost!
Portia, adieu. I have too grieved a heart
To take a tedious leave. Thus losers part. (2.7.81-83)
Thus, the Prince of Morocco demonstrates strong character in the end, as he makes no complaints.