In Act one of the play, Othello is presented to us as a noble Moor, a man of royal descent, confident and eloquent. He is proud, a man of authority and vastly skilled in the art of war. His experience and military prowess earn him the respect of the Venetian Senate, so much so, that they entrust him with the great task of protecting the security of the state by appointing him general of the Venetian army, even though he is a foreigner. Their respect for him is further emphasised when they request his presence in Cyprus to ward off a threat by the Turkish army. Othello is loyal to the Venetian state, a man of integrity and honour.
It is therefore tragic that this highly respected general has some fatal flaws which are gradually exposed and exploited by the Machiavellian Iago. We soon discover that Othello, as an outsider, is uncomfortable in Venice. This makes him insecure, naive and gullible. Since he does not entirely understand Venetian custom, he easily believes what Iago tells him about the rudiments of behaviour in Venice. He becomes fodder for Iago's manipulation, naively believing what he is told. Othello trusts Iago emphatically, a trust that Iago gleefully abuses.
We also learn, through Iago's machinations, that Othello is insecure about the age difference between he and Desdemona, his foreign status, and, to some extent, his race. Iago's exploitation of these flaws is so complete that he turns Othello into a jealous monster intent on revenge who, in an act of misplaced loyalty, destroys the one he loves for, what he believes, is the protection of others. All these factors lead to his fall from grace and final destruction.
Iago is an experienced and skilled soldier and Othello entrusts him with the duty of ensign. He is highly intelligent, worldly-wise and eloquent. He is one who is easily trusted - clearly displayed by the faith characters have in him. It is apparent throughout, that Iago possesses an uncanny understanding of human behaviour. He is keenly observant, has a quick wit and can 'think on his feet'. These are the qualities which allow Iago to easily gain the respect of others.
Conversely though, Iago is a bitter, cynical man, intent on revenge, no matter what the cost. He is amoral, duplicitous, scheming, a master manipulator. Iago is the essence of evil. He purposefully and slyly plots the downfall of others, ruthlessly and without remorse. It is clear that he despises the good in others and often comments about his dislike for these qualities, specifically when he refers to Othello, Desdemona and Cassio.
Iago has no qualms in bringing about the downfall of those he despises, for no particular reason. Iago has no true motive for the evil he commits - a truly Machiavellian character. He displays misogynistic tendencies when speaking about and dealing with women. It is this cornucopia of evil within Iago that compels him to plot the destruction of others and bring about his own doom.
Desdemona is a true innocent. She is a loving, caring, modest young woman who would go out of her way to help others, as is shown by her urgent attempts at trying to help Cassio regain his position. She is truly loyal to her husband and dotes on him. At the beginning of the play we learn of her strength when she opposes her father and declares loyalty to Othello. She also later scolds Othello, telling him that she did not deserve his harsh treatment.
Possibly because of custom, Desdemona appears subservient and docile to Othello and meekly bows to his authority. She is saintly, so much so, that even when smothered by Othello, she takes responsibility. One could say that it was Desdemona's goodness and her implicit belief in the goodness of others, that led to her demise. She could not fathom that there lurked within her husband or Iago such pernicious evil and she unresistingly acquiesced to her death.
Emilia, Iago's wife and Desdemona's maidservant is an intelligent, spirited woman, absolutely devoted to Desdemona. She is well-versed in Venetian custom, is pragmatic, somewhat bawdy and worldly-wise. Her greatest mistake is probably the fact that she is so loyal to her husband. In this regard then, she adheres to the expectations of Venetian custom - that a wife should be obedient and loyal to her husband. She trusts him so much that she, without hesitation, procures him Desdemona's handkerchief, not even questioning his motives and accepting his dismissive retort.
Emilia is the first to realise what evil Iago has committed and she courageously confronts and implicates him, which leads to her death at his hand.
Overall, the characters sense of pride and self-doubt lead to the tragic ending in the play. Othello does not have enough pride in himself to believe that his wife could remain faithful to him, and the doubt that this creates leads him to believe Iago and blame Desdemona for having an affair with Cassio.
Iago has too much pride and does not see why Cassio should have gotten the title of lieutenant instead of him. He doubts his own strength as an officer and rather than accepting Othello's decision, Iago decides to seek revenge.
Desdemona does not understand her husband and cannot understand why he would have such thoughts about her being unfaithful.
And Emilia tries to be a good wife to Iago by doing his bidding and only comes to her senses after it is much too late.