The Merchant of Venice is named Antonio, a prominant figure in Italy and a honest, upstanding citizen. His role in the play is major, as he wishes to help his bankrupt friend Bassanio with the money needed to woo a wealthy heiress, Portia. However, Antonio has no liquid capital, and so he secures a loan from the money-lender Shylock, leaving the infamous "pound of flesh" listed as security. Antonio's subsequent monetary troubles drive the story as Shylock attempts to collect his due.
I am as like to call thee so again,
To spit on thee again, to spurn thee too.
If thou wilt lend this money, lend it not
As to thy friends; for when did friendship take
A breed for barren metal of his friend?
But lend it rather to thine enemy,
Who, if he break, thou mayst with better face
Exact the penalty.
(Shakespeare, The Merchant of Venice, shakespeare.mit.edu)
This altruism on Antonio's part shows how deeply he values his true friendship with Bassanio, and his willingness to undergo harm shows how little he cares for his own concerns when others may suffer. Shylock's case against Antonio is overturned when Portia, in disguise, proclaims before the Duke's court that the stipulation was for flesh alone, not blood, and since he cannot have one without the other his claim is void. However, despite his almost-saintly patience and compassion, Antonio is not entirely "good," or morally ethical; even after winning the case, he uses the Duke to force Shylock into religious conversion in revenge for the emotional distress of the case.
As Professor Szatek(In the book THE MERCHANT OF VENICE, NEW CRITICAL ESSAYS, 2002) noted, the opening dialogue "clearly identifies Antonio as a merchant." In the court scene(4.1), Portia asks: "Which is the merchant here, and which the Jew?" I think that Antonio is the only character explicitly identified as a merchant. Some have suggested that Shylock seems to be the chief protaganist: He must deal with Antonio's offences, his daughter's elopement, and as we find in Act 2, scene 5, the absence of his wife. In his aside that begins "How like a fawning publican he looks!"(1.3.38 or so), his inner thoughts are also presented. Also, as we have seen, one might think that the title should refer to more than one character.
One reason this question occurs is that many regard Portia and Shylock as the two principal characters, and the means by which Portia's father gained his wealth, I think, is not revealed. Therefore, one might suppose that she is the merchant.