As a tragic hero, Othello is unusual in that he is relatively passive and reactionary. His action does not drive the plot. He is known as a man of action—a military leader, and a successful one. His eloquent words and valiant actions in battle have given him a reputation. But in the play, he does not take the most active role in his own character arc. Iago, in contrast, manipulates behind the scenes from the first moments of the play. Othello wins Desdemona by his stories of the adventures he's experienced, and he seems to be more in love with the confidence she gives him by her adoration than with the woman herself. He is not present when the others arrive at Cyprus, and he arrives at the celebration just in time to discipline Cassio. He is more acted upon than actor. In one reading of the play, Iago can be characterized as a Satan figure, and this, along with the "temptation" that takes place in the castle gardens on Cyprus, puts Othello in the role of Adam.
Iago declares early on in the play, "I am not what I am." And indeed, the audience might well leave the play wondering, Who is Iago? What is his motivation for causing such utter destruction in the lives of arguably innocent people? Is he angry at being passed up for a promotion? Does he think Emilia has been sleeping with Othello? These petty motivations seem inadequate in light of the absolute thoroughness with which Iago manipulates the people around him. He is excellent at immediately assessing a person's strengths and how these strengths can be turned into weaknesses. He exploits Roderigo's desire for Desdemona and his generous hand with money. He leverages Cassio's desire to please Othello. He turns Desdemona's kindness to Cassio against both herself and Othello. And he keys in on Othello's love for Desdemona as the one weak point in Othello's armor. If Othello is an Adam, Iago is the snake in the garden: Satan himself.
Desdemona is a woman of strength and purpose. She falls in love with Othello and acts on her own to elope with him, eschewing social conventions. She argues convincingly to go with him on the military mission to Cyprus. And she is unwavering in her kindness to Cassio. Through no fault of her own, her independent spirit and goodness are used against her by Iago in his plot to ruin Othello. Yet her actions on her deathbed are curious. She seems to take the blame for her own death, and seems quite passive as she is murdered. These contradictions may suggest Desdemona's independent spirit is at odds with the submissive obedience required of her by society.
Michael Cassio has recently been appointed as Othello's second in command, much to the chagrin of Iago: "that never set a squadron in the field,/Nor the division of a battle knows." Cassio's flaws open him up to exploitation by Iago, who seeks revenge against Cassio for this appointment. Cassio is more brains than brawn, and he doesn't hold his alcohol well. He's also a flirt, and he tends to characterize the women in his life as Madonnas (Desdemona) or whores (girlfriend Bianca).
In love with Desdemona, Roderigo is wealthy and foolish. He provides a steady flow of income to Iago, who has promised to help Roderigo woo Desdemona. His desperation makes him susceptible to Iago's manipulations, and Iago convinces Roderigo to help kill Cassio, who has been positioned by Iago as a rival suitor for Desdemona. In the end, Iago kills Roderigo.