What sort of person is Cassio? In Act II, what happens to him, and how does Iago plan to use the situation in his plan against Othello?
We do not learn very much about Cassio over the course of the play. A pawn in Iago's game, we know that he is a Florentine, rather than a Venetian, and that Iago thinks he is too inexperienced to have been granted the rank of lieutenant. According to Iago, he "never set a squadron in the field" and is rather bookish, "a great arithmetician." However, we also know that Othello, renowned as a general, saw some virtue in him, having selected Cassio specifically as his officer. Cassio is a man of seemingly better "breeding" than Iago, but Iago rightly interprets that his courtesy covers a polite, naive nature which will help Iago to "ensnare" him.
Iago knows that Othello thinks well of Cassio, and he must also know, beneath his jealousy, that Othello has good reason to do so—which is why he sets himself to destroying Cassio's reputation in act 2 in order to further his greater plan. Iago observes that Cassio is courteous and intimate with Desdemona; he is determined to use this against him. First, however, he must change Othello's opinion of his officer—"abuse him to the Moor in the rank garb"—so that Othello will believe what Iago tells him about Cassio and Desdemona.
When, in act 2, scene 3, Othello leaves Cassio to the guard, he is trusting him to oversee what is going on: he believes that Cassio is trustworthy. However, although Cassio tells Iago he has "poor and unhappy brains for drinking," he allows Iago to pressure him into drinking more. He quickly becomes belligerent, as Iago had hoped, and engages in a fight with Roderigo. When Othello returns to find his lieutenant drunk and brawling, he is very disappointed in him and removes him from his position, to Cassio's deep sorrow—"I have lost the immortal part of myself, and what remains is bestial," Cassio says, lamenting the loss of his reputation.
Thus, Iago has set the stage for the next part of his plan, in which he will convince Othello that Cassio, no longer the paragon in Othello's mind which he once was, is sleeping with Desdemona.
One of the interesting things about the character of Cassio is that Shakespeare reveals it to us through the way others see him, almost as if he is just a reflection of what people want him to be. Iago's impression of him is the first one we get, but Iago sees him through eyes clouded by jealousy. Cassio has gotten the promotion Iago thinks should be his, so he is scornful of Cassio's ability. He sees Cassio as a brown-noser and not a real soldier. Cassio is an outsider and a newcomer. To Desdemona, Cassio is a loyal friend; to Othello he is a fine choice for Lieutenant.
Iago sets up Cassio to get into a drunken sword fight with his henchman Roderigo, hoping to get him killed. When Roderigo proves too cowardly to kill Cassio, Iago uses the opportunity to get Cassi demoted and then puts the idea into Othello's head that Cassio is having an affair with Desdemona. Cassio appears to be oblivious to all of this plotting throughout the entire play and is as surprised as everyone else when he finds out the truth about Iago. Cassio is a linchpin to Iago's plans, but he is not important in and of himself.