Iago serves as Othello's ensign. When Othello promotes Cassio instead of him, Iago feels wronged and plans revenge against both men. He manipulates Cassio into tarnishing his own reputation and persuades Roderigo to attempt to kill Cassio. When this plot fails, Iago kills Roderigo to avoid being exposed. He convinces Othello that Desdemona is unfaithful and manipulates him into murdering her. Subsequently, Iago kills his own wife, Emilia, and is arrested by Cassio at the play's conclusion.

Iago is a seasoned soldier with extensive battle experience, having fought alongside Othello in both Rhodes and Cyprus. He is also one of Shakespeare's most notorious villains. A master manipulator, Iago expertly influences the actions of other characters to serve his own ends. He understands what drives them and uses this knowledge to his advantage. For instance, he exploits Roderigo's desire to be with Desdemona to manipulate him. Cassio, concerned with maintaining his public image, readily follows Iago's advice to seek Desdemona's help in restoring his rank. Iago also takes advantage of Desdemona's kind and generous nature, knowing she will eagerly try to reconcile her husband with his lieutenant. Finally, he preys on Othello's jealousy and insecurities to convince him of Desdemona's infidelity. Emilia, however, remains immune to his manipulation, likely because she knows him too well.

Iago's scheme involves getting Cassio demoted from his position as lieutenant and then suggesting that Cassio ask Desdemona to plead with Othello on his behalf. Desdemona does so, which fuels Othello's suspicions. Othello's distrust begins when Iago points out that Cassio left hastily as they approached. Iago also reminds Othello that Desdemona deceived her father when she eloped with Othello, indicating her capacity for deceit. Furthermore, Iago highlights the differences in color, age, and social status between Othello and Desdemona. He also uses the handkerchief Othello gave Desdemona as a love token to suggest her guilt, a situation orchestrated by Iago.

Iago provides several hints to the audience regarding his motives. Primarily, he resents not being chosen as Othello's lieutenant. He expresses this to Roderigo:

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 frustrated about being overlooked for a promotion and sees an opportunity to retaliate against both Othello, who has wronged him, and Cassio, who represents that slight. Additionally, he suspects that Othello has been unfaithful with his wife, Emilia. He references this suspicion twice: "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)

Iago is evidently so troubled by the idea of Emilia being with Othello that he has accused her of the act. Emilia, as usual, dismisses the accusation as ridiculous (IV.ii.145-147). In their baseless jealousy, Iago and Othello share a significant similarity.

Iago and Othello share another common trait. At the play's conclusion, when Othello is detained and Iago is brought before him, Othello remarks, "I look down towards his feet; but that's a fable" (V.ii.287). He is checking to see if Iago has cloven feet like the devil he now believes him to be. Despite Iago's hatred for Othello and Othello's newfound disdain for Iago, they are quite similar in feeling excluded from the elite Venetian society. When Othello calls him "honest, honest Iago" (V.ii.155), it implies more than just honesty in words. Iago is the only one who directly addresses Othello's sense of inadequacy, a feeling Iago might also share. At the play's end, after killing Roderigo and Emilia and confessing his actions, Iago is captured.

If anything, Iago is an even more formidable character than Othello. Unlike the internally conflicted Moor, Iago is confident and unwavering in his actions and self-assessments. For practical reasons, Iago pretends to be what he is not—a loyal supporter of Othello with limited ability to aid his "friend." But unlike any other character in Shakespeare's works, Iago openly admits to being a villain driven solely by hatred for his superior. It is Iago's unyielding, maniacal obsession that gives his lines, like his casual remark, "I am a very villain else" (IV.i.126), their sharp impact. Before piecing everything together, Iago's wife comforts a distressed Desdemona, who is reeling from her husband's inexplicable outburst:

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)

Iago's wife, who knows him best, provides the most accurate description of him. She declares that the person behind Othello's fury is an "eternal villain." Despite his vast ability to create evil, he is ultimately a small-minded individual driven by petty resentments.

No matter how much Iago is responsible for Othello's downfall, he remains one of Shakespeare's most notorious villains. He is variously seen as a clever opportunist who seizes available chances, a personification of evil, or a typical "devil" or "vice" figure. Iago's true motivation is a subject of much debate. Throughout the play, he offers various reasons for his actions—anger at being overlooked for promotion, suspicions about Othello and Emilia, and a desire for Desdemona. Yet, his plans seem oddly incomplete, as if he is inventing both his schemes and motivations on the fly. The renowned 19th-century writer and critic Samuel Taylor Coleridge described this as the "motive-hunting of a motiveless malignity." For many readers, Iago's actions are simply evil, defying explanation or understanding. Others see no need for explanation, viewing Iago as a typical dramatic villain. However, many scholars consider Iago a more fully developed character, both emotionally and psychologically complex. These critics argue that his pride, desire for power and control, brilliant scheming, and jealousy make Iago a captivating, multi-dimensional figure.