On one hand, Antonio can be characterized by his kindness. He lends his friend Bassanio money and agrees to the stipulations of Shylock's agreement. By the standards of the time, Antonio is also considered generous because as a Christian he lends money and does not charge interest. This also supports the idea that he is kind. On the other hand, Shylock suffers from Antonio's condemnation of him and his being Jewish.
He is also somewhat melancholy or sad. His initial sadness at the beginning of the play goes unresolved. The reader never learns why he is sad. Some interpretations say he is sad because he is uneasy by his choice of profession - namely a merchant whose prime preoccupation would be to earn money. Other modern interpretations suggest that he is in love with Bassano and that is his motivation for lending him money, yet also the cause of his sadness.
You cannot read Antonio as some who was purely kind and virtuous. Shylock has suffered under his condemnation and he spits at Shylock and calls him names. Here Antonio shows himself to be a hypocrite because although he does not charge interest, his profession is the same as Shylock's.
For a lengthier description, look under the enotes link.