Michael Cassio is Othello's lieutenant, the man who got the post Iago wanted and one of the targets of his plan. In the end, he is the one left in command of Cyprus.
In the first scene, Iago describes him to Roderigo as "a Florentine" who "never led a squadron in the field". In Act II, Scene 3, Othello puts him on watch in Cyprus, and Iago persuades him to drink, though knowing he has no head for wine. Roderigo provokes him, Cassio wounds Montano, and Othello dismisses him on the spot. Alone with Iago, he laments: "Reputation, reputation, reputation! O, I have lost my reputation!"
Following Iago's advice, he asks Desdemona to intercede for him, and it is this request that Iago turns into proof. In Act III, Scene 3, Othello confirms to Iago that Cassio knew of his love for Desdemona when he was courting her, "from first to last"; Cassio leaves in haste on seeing Othello arrive, and Iago makes use of the gesture. Cassio finds a handkerchief in his own room ("I found it in my chamber") and gives it to Bianca to copy the pattern. In Act IV, Scene 1, Othello, hidden, takes Cassio's laughter about Bianca for a confession about Desdemona.
In Act V, Roderigo attacks him in the street and Iago wounds him in the leg from behind. Cassio says to Othello: "Dear general, I never gave you cause." And Lodovico declares: "And Cassio governs Cyprus."