Othello's wife Desdemona.
Cassio was Othello's lieutenant - his second in command.
He is Othello's standard bearer and third ranking officer. Othello as General with Cassio as Lieutenant
Neither. Othello was the captain, Cassio the lieutenant and Iago the ensign.
Lieutenant.
Roderigo stabs Cassio. Of course, at the time he does so, Cassio is no longer Othello's lieutenant, since Othello has fired him and hired Iago instead. And nobody stabs Iago, unfortunately.
Lieutenant.
Iago tells Cassio that he must persuade Othello to reinstate him. He says "the general's wife is now our general" and so the best way to persuade Othello is to get Desdemona to plead on Cassio's behalf. Since Iago has also told Othello that Desdemona and Cassio are having an affair, the harder Desdemona pleads, the worse it is going to look for both her and Cassio.
He says that Othello placed Cassio as his lieutenant instead of him.
Othello promoted Cassio as his lieutenant instead of Iago.
He says that Othello placed Cassio as his lieutenant instead of him.
Both. In a way. He is jealous of Othello because he has everything he wants: a high rank and a beautiful wife from a upper class family. He is jealous of Cassio because Othello has promoted him to Lieutenant and not him.
Iago suggests to Othello by counter-suggestion that his wife is having an affair with his lieutenant Cassio.