Who was cassio ? What connection had he with Othello and Desdemona ?

Othello's lieutenant, though he has little field experience. Cassio is a smooth-talking Venetian courtier, the opposite of Othello in many respects, which is why Othello admires him. Cassio is close with Desdemona but they are only friends. This sets up Iago's plan to convince Othello that the two are having an affair.