In 'The Merchant of Venice', the Christian merchant who borrows from Shylock is Antonio. He borrows—or, more precisely, guarantees—a loan arranged to help his friend Bassanio court Portia.
Antonio is wealthy and respected in Venice, but his money is tied up in merchant ships at sea. Because he cannot provide Bassanio with ready cash, Bassanio approaches Shylock for a loan of 3,000 ducats, with Antonio standing as guarantor. Shylock proposes the notorious penalty: a pound of Antonio's flesh if the debt is not repaid on time.
When Antonio's ships are reported lost, Shylock demands enforcement of the bond. The courtroom scene then brings together the play's central issues: law, mercy, friendship, religious prejudice, and financial risk. Portia, disguised as a lawyer, ultimately finds a legal technicality that prevents Shylock from taking the flesh.
Bassanio is the friend seeking money and Portia's hand; he is not the merchant. Lorenzo and Gratiano are also Christian characters, but neither is the debtor at the center of the bond plot.