I have to discuss Emilia here as a main character, as well. She is an integral part of the play and she is a very intriguing character, as well. Emilia, in my opinion, is neither clearly good or evil, as sullymonster mentioned above about Othello. She clearly cares deeply for Desdemona, whom she works for, but she also knows how to play coy with Iago, who grabs Desdemona's handkerchief from Emilia's grasp after she finds it. What bothers me about this is that Emilia should have given the handkerchief to Desdemona immediately and the play would've turned out differently. In the end, Emilia "goes to bat" for Desdemona and turns against Iago; however, she suffers for it...she is murdered by Iago.
Iago is the most clear example of evil in the story. He is motivated by jealousy and manipulates people with no concern for the consequences. He plots the murder of Desdemona with no regard for human life. He even says of himself, ""I am a very villain."
Othello is not clearly good or evil. Though he expresses nobility and takes responsibility for his ill-fated actions, Othello is plagued by fatal flaws. He has a jealous heart and is prone to over-reaction. He is so tempermental that he is able to commit murder, believing that it has been "earned" by an unfaithful wife. In many ways, he is a mirror of Iago. What makes him different is the respect for honesty and human life that he does show.
Desdemona is innocent and good. She trusts fully and loves completely. She does show dishonesty in her elopement, but as love was the driving force and her father in a position of tyranny, it is possible to say she is still good. This is what makes Iago's plan and Othello's acts so heinous - they are against an object of purity.
I am not sure that Desdemona should be painted as being pure and representing goodness in the play. Certainly there are elements of that, but she is also slightly selfish in her treatment of other characters, particularly Othello. The fact that she chooses to naively and blindly beleif in him, places a great deal of expectation on his shoulders and makes him highly insecure. In fact it could be argued that her unwillingness to accept othello outside her romantic ideal of him contributed to both her own and his death.
Is Desdemona really good? Her character can be interpreted in different ways and critics have viewed the play from different views. How would the audience's reaction be towards Desdemona if she really was cheating on Othello with Cassio. Some arguments that these critics would give are:
In Act 3, Scene 3 when Iago says: "She did deceive her father, marrying you;/And when she seemed to shake, and fear your looks,/She loved them most." (l. 205-207) This indicates that if she coud deceive her father, she could also deceive Othello.
Also, Cassio constantly refers to Desdemona as "divine" (2:1 l. 73) and talks about her as if she is a goddess.
However, you don't have to agree with this. I personally believe that she was innocent, but this aspect gives the story an interesting twist.