Iago is Othello's ancient, or ensign. When Othello promotes Cassio, Iago feels slighted and plots revenge against them both. He manipulates Cassio into discrediting himself and urges Roderigo to slay Cassio. When the plot fails, he kills Roderigo to keep from being exposed. Iago convinces Othello of Desdemona's unfaithfulness and maneuvers him into killing her. He then murders his own wife, Emilia, and is taken into custody by Cassio at the play's end.

Iago is a soldier with a good deal of experience in battle, having been on the field with Othello at both Rhodes and Cyprus. He is also one of Shakespeare's greatest villains. He is a master manipulator of people and gets the other characters in the play to do just what he wants. He manipulates others through a keen understanding he seems to have of what motivates them. For example, Iago uses the vision Roderigo has of a union with Desdemona to manipulate Roderigo. Cassio is a man driven by the need to maintain outer appearances, and he easily accepts Iago's advice that he recover his rank by going through Desdemona. Iago also uses to his advantage the fact that Desdemona is of a kind and generous nature, one who will gladly accept the opportunity to persuade her husband to make amends with his lieutenant. And, finally, Iago uses Othello's jealous nature and his apparent insecurity to convince Othello of Desdemona's infidelity. Emilia is the only one, it seems, that Iago cannot manipulate, perhaps because she knows him so well.

Iago schemes to have Cassio demoted from his post as lieutenant, next suggesting that Cassio ask Desdemona to intercede for him with Othello on his behalf. She does, which contributes to Othello's suspicions. Othello first begins to distrust Desdemona when Iago points out that, as he and Othello approached Desdemona and Cassio, Cassio quickly departed. Iago also reminds Othello that Desdemona, in eloping with Othello, deceived her father, which shows her capacity for deception. Additionally, Iago reminds Othello of the differences between Othello and Desdemona in terms of color, age, and social status. The handkerchief that Othello had given to Desdemona as a love token is also used to indicate her guilt, a situation also engineered by Iago.

Iago provides the audience with a number of clues to the motives for his actions. First, he feels a certain rancor at not being chosen as Othello's lieutenant. He reassures Roderigo of this:

Preferment goes by letter and affection,
And not by old gradation, where each second
Stood heir to th' first. Now, sir, be judge yourself
Whether I in any just term am affin'd
To love the Moor.
(I.i.36-40)

He is disgruntled at having been passed over for promotion, and he sees a chance to get back at both Othello, who has slighted him, and Cassio, the mocking symbol of that slight. Second, he suspects that Othello has engaged in adultery with his wife, Emilia. He mentions this on two occasions: "I hate the Moor, / And it is thought abroad that 'twixt my sheets / He has done my office" (I.iii.388-390), and

I do suspect the lusty Moor
Hath leap'd into my seat; the thought whereof
Doth (like a poisonous mineral) gnaw my inwards;
And nothing can or shall content my soul
Till I am evened with him, wife for wife.
(II.i.299-303)

Apparently, Iago is so distressed by the thought of Emilia sleeping with Othello that he has accused Emilia of the act. As is typical of her, Emilia characterizes the accusation as absurd (IV.ii.145-147). In their unfounded jealousy, Iago and Othello are very much alike.

Iago and Othello are alike in another way as well. At the end of the play, when Othello is under arrest and Iago has been apprehended and is brought into his presence, Othello says, "I look down towards his feet; but that's a fable" (V.ii.287). He is looking to see if Iago has cloven feet like the devil Othello now thinks him to be. But for all of Iago's hatred of Othello and Othello's newly discovered contempt for Iago, the two are very much alike in their sense of being excluded from upper-class Venetian society. When Othello calls him honest, honest Iago" (V.ii.155), he speaks of more than verbal truth. Iago is the only character who speaks directly to Othello's sense of his own inadequacy, a sense of inadequacy Iago perhaps shares. At the end of the play, after killing Roderigo and Emilia and revealing all he has done, Iago is taken prisoner.

If anything, Iago is an even stronger character than Othello. Unlike the internally-torn Moor, Iago is certain and entirely consistent in his acts and in his self-appraisals. For the sake of expediency, of course, Iago shows himself to be what he is not, a loyal supporter of Othello with limited capacity to help his "friend." But more than any other character in Shakespeare's plays, Iago is a self-professed villain whose sole motive is hatred toward his superior. It is the unrelievedly evil, maniacal fixation of Iago that provides such lines as his off-hand comment, "I am a very villain else" (IV.i.126), with their acute pungency. Before piecing it together, Iago's wife consoles a distraught Desdemona reeling from her husband's inexplicable tirade:

I will be hang'd if some eternal villain,
Some busy and insinuating rogue,
Some cogging, cozening slave, to get some office,
Have not devis'd this slander.
(IV.ii.130-133)

The person who is most intimate with Iago, his wife, furnishes the most accurate account of him in declaring the person behind Othello's rage to be an "eternal villain," who is large in his capacity to concoct evil but ultimately a small-minded slave acting on a relatively petty resentment.

Regardless of the degree to which Iago is to blame for Othello's downfall, he remains one of Shakespeare's most villainous creations, variously described as a brilliant opportunist taking advantage of the chances presented to him, as a personification of evil, and as a stock "devil" or "vice" figure. Iago's motivation remains a topic of considerable debate. Although he offers numerous motives throughout the play—resentment at being passed over for promotion, suspicions about Othello and Emilia, desire for Desdemona—Iago's plans seem curiously incomplete; he appears to be making up both his schemes and his motives for them as he goes along. The noted nineteenth-century writer and critic Samuel Taylor Coleridge described this process as the "motive-hunting of a motiveless malignity," and for many readers Iago's behavior is simply evil, beyond explanation or understanding. For others, no explanation is necessary. They consider Iago a devil or vice figure, a stock dramatic villain. Many scholars, however, find Iago a more fully-out character, emotionally and psychologically complex. According to these critics, his pride and desire for power and control, along with his brilliant scheming and his jealousy, make Iago a fascinating, multi-faceted figure.