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.