Shylock's character in Merchant of Venice is very complicated, especially in today's post-holocaust age. First, during the beginning of the play, Shylock's personality traits include very negative things such as greed and vindictiveness, which go along with the point of view of Antonio, Bessanio, Portia, and Gratiano, as well as Shylock's daughter who flees the Jewish community for love. Shakespeare used the source play The Jew of Malta, however, he portrays a must more three-dimensional character in Shylock than is portrayed in Malta. During the trial of Antonio and the whole "pound of flesh" bit, the audience doesn't see much other than the vindictiveness in Shylock's character, which is chocked up to an overweening focus on justice. Shylock's focus on "justice" as he sees it neglects the need for mercy. Of course, Portia's great "The quality of mercy" speech (Act 4, Scene 1) is an answer to this focus on justice alone.
However, you have to remember that Shylock is not just a mustache-twirling-villainous picture of antisemitism. Shylock's character is actually a lot deeper than that. One of Shylock's greatest speeches is the "Hath not a Jew eyes" speech (Act 3, Scene 1), which calls the audience and the other characters to a remembrance of their common humanity. At the end of it all, Portia's mercy speech wins out in Antonio's favor, but not in Shylock's. The forcing of Shylock's "conversion" to Christianity, actually ostracizes him from both the Jewish community he belongs to and the Christian community, since he's not a true convert and is from a Jewish background. At the end of it all, Shakespeare has to abandon Shylock in Act 4 to maintain the comedy as a comedy. So, as far as Shylock's characteristics, Shakespeare shows a range of traits from vindictiveness, justice-loving, greed (as evidenced in his usury), all the way to his love for his daughter and need for a community that he is ultimately deprived of.
If you're interested in an interesting look at Shylock's character, I would suggest the newer movie rendition of Merchant, which came out in 2004. It gives Shylock depth which his counterpart character in The Jew of Malta would never have.