Cassio is a man of honor, believing in the order of things. He believes that a man's reputation is crucial to his well-being and is chivalrous, trustworthy, generous and apparently good-looking. His popularity with the womenfolk is pure and out of respect for them not because he lusts after them.
Unfortunately one of his weaknesses is his relationship with Bianca, a woman of doubful morals, a prostitute who does appear to be wholly in love with Cassio although he does not feel as strongly.
Cassio's character does not have a lot of depth and his main function in the play is to further Othello's jealousy and eroding trust. He is easily manipulated due to his trusting nature and in Act II iii Iago takes full advantage when he persuades Cassio to drink more than he knows he can hold:
I have very poor and unhappy brains for drinking
He does not suspect Iago's intentions and becomes brazen after another drink and even refers to the fact that "the Lieutenant is to be saved before the Ancient" rather more because it is the order of things and due to his naiivety than because he is better than Iago. .
Cassio is perhaps referring to a commonplace for maintaining military order, but the implication is that Cassio is superior by virtue of his title alone
Of course Iago can fuel his own hatred because of this statement and justify his belief that Cassio does not deserve the position of lieutenant - not even being a skilled soldier. Iago's initial need to avenge himself against Cassio who was given the position he believes he deserves more is reinforced and Iago can use Cassio to destroy Othello and Desdemona at the same time.
Iago goes on in this scene to refer to Cassio's inability to manage his drinking as if it is a regular occurence sufficiently that Othello should be warned. He refers to it as "his infirmity." He wants others to see Cassio's "vices" as Cassio appears far too perfect otherwise.
Iago is also able to use the situation and Cassio's inability to control himself to further his plot against Othello. Others will question Othello's leadership qualities if he has appointed such an inconsistent lieutenant.
Cassio has "lost the immortal part of myself" by getting involved in a brawl and is sorry for his part but he also walks straight into Iago's trap and actually trusts Iago enought o discuss his situation with him again showing his naiivety.
It seems that none of the characters are a match for Iago!