Cassio is chosen over Iago to be Othello's lieutenant. He is discredited when he participates in a drunken brawl during Othello's wedding celebration. Cassio survives a murder attempt by Roderigo, wounding his attacker, and is appointed deputy governor of Cyprus after Othello is recalled to Venice.

According to Iago, Cassio is "a great arithmetician" (I.i.19), one "That never set a squadron in the field" (I.i.22). Cassio knows battle only from books, unlike Iago who has had a good deal of experience in combat. Cassio is apparently a handsome man, and the ladies are attracted to him. But Cassio also has his weaknesses. When Iago tries to get him to have a drink in celebration of the Turks' defeat and Othello's marriage, Cassio says, "I have very poor and unhappy brains for drinking" (II.iii.30-31). Cassio is the perfect dupe for Iago. Cassio is attractive, and this fact encourages Othello's belief in Iago's suggestion that Desdemona desires Cassio. Cassio's inability to drink also gives Iago another weapon in his plan to abuse both Cassio and Othello.

Cassio represents the class privilege of which Iago is so envious and resentful. It rankles Iago that Cassio seems to have bought into the idea that he is socially superior. When they are drinking together, Cassio tells Iago that "the lieutenant is to be sav'd before the ancient" (II.iii.105-106). Cassio is perhaps referring to a commonplace for maintaining military order, but the implication is that Cassio is superior by virtue of his title alone. Again, when Othello disgraces Cassio by scolding him in public and stripping him of his rank for neglecting his watch and brawling with Montano, Cassio laments most the loss of his reputation. In his great desire to regain that reputation, he plays right into the hands of Iago, who suggests that Cassio appeal to Desdemona to intervene with Othello for restoring his rank. For Iago, through whose eyes the audience gets its only sense of Cassio's character, Cassio is all reputation and title with no real substance. Iago refers to "One Michael Cassio, a Florentine" (I.i.20) while a gentleman in Cyprus refers to "A Veronesa; Michael Cassio" (II.i.26). Perhaps Cassio has no inner qualities that identify who and what he is, only his titles. Even so, he ends up in charge of the Venetian troops in Cyprus.