He is a Jewish moneylender who earns his living by charging interest on money he loans (like modern banks). He often speaks prose in the play, which marks him out as an outsider.
He is persecuted by all the non-Jews he knows:
He tells Antonio, "suff'rance is the badge of all our tribe".
He is verbally abused and bullied by most characters in the play and is called cruel names including "villain with a smiling cheek, cut-throat dog, bloody creditor, damned inexecrable dog".
He is clearly an intelligent businessman:
He is very astute and is aware of other people's concerns - he knows all about Antonio's business ventures.
The main reason he hates Antonio is financial: "I hate him for he is a Christian; / But more, for that in low simplicity / He lends out money gratis".
So, why does he make such a strange agreement with Antonio, asking for a pound of Antonio's flesh instead of interest if the loan is not repaid within the arranged time? Does he genuinely want to be friends with Antonio when he says "I would be friends with you, and have your love"?
He is Jessica's father. His daughter hates him and calls him a "devil". We see him being impatient with her and ordering her around. When she runs away, he seems as upset about the loss of his money as her: "My daughter! O my ducats! O my daughter! ... My ducats and my daughter!"
It would seem that Shylock cares for money more than anything else. Yet in an often forgotten moment when Shylock finds out his daughter traded his wife's ring for a monkey he cares only for its sentimental value. "Out upon her! Thou torturest me, Tubal: it was my turquoise; / I had it of Leah when I was a bachelor: / I would not have given it for a wilderness of monkeys".
He wants revenge:
Perhaps it is the loss of Jessica as well as all the harsh treatment he has suffered from Antonio - and others - over the years that makes him bitter enough to ask for Antonio's pound of flesh.
He cleverly argues in Act III, scene 1