The term CASSIO'S RIVAL is represented by the four letter word IAGO. Iago is a character in William Shakespeare's play Othello, and is the main antagonist. He is a scheming, manipulative villain who plots to ruin the life of the protagonist, Othello, by making him believe his wife Desdemona is unfaithful. Iago is Cassio's rival because he is jealous of Cassio's promotion to lieutenant and plots to have him removed from his position.