Initially, in The Merchant of Venice, Bassanio is, by his own admittance, a shallow character who has squandered his wealth - "disabled my estate" (I.i.123) - and amassed substantial debt. He has already borrowed from his friends and now wants to borrow some more because he has a plan to "get clear of all the debts I owe."(134) He proposes to be a suitor to Portia "a lady richly left" (161) but in order to "hold a rival place"(174) with her other suitors, he needs to show his own value.
Portia must find a suitor in terms of her late father's will and Bassanio is certain that she has communicated "speechless messages" (164) to him, making hm keen to marry her. Antonio is more than willing to loan monies to Bassanio but as his wealth is currently tied up in ships, Bassanio will have to "try what my credit can in Venice do"(180). Bassanio is able to secure a loan from Shylock.
Shylock and Antonio have an age old feud - Jew versus Christian - and Shylock does not hide his contempt - "I hate him for he is a Christian" (I.ii.37) nor Antonio who will "spit on thee again."(125) After a heated exchange a deal is struck and Antonio, if he cannot repay Shylock, will be required to give "an equal pound of your fair flesh" (144-45) in order to settle the bond.
Bassanio realizes that this is too much to ask and would rather "dwell in my necessity"(150) revealing the side of his character that the audience will see develop as Bassanio pursues Portia and comes to love her sincerely. Bassanio is uncomfortable with these not so "fair terms and a villain's mind"(174) but is persuaded by Antonio to proceed as Antonio is confident that the bond will be repaid long before it is due, when his ships come into port.
Hence, we see the beginnings of Bassanio's mature self developing who does not want to take advantage of his friend and is cautious of trusting the volatile relationship between Antonio and Shylock.