When we begin, Cassio is one of Othello's soldiers, and is recently appointed the general's second-in-command. This infuriates Iago, as he wanted to be lieutenant, and Cassio is a math (not muscle) guy, so Iago cannot understand this appointment.
Like all people, real and imagined, he's got some flaws. First, he's a lightweight when it comes to drinking. This is the weakness that Iago exploits (when Iago gets Cassio drunk and sends him off to fight Roderigo). Second, Cassio's a little too much of a lady's man. This angers Iago, as Cassio's kissing Emilia in front of Iago is a bad idea. It also comes back to bite Cassio in the end, since his flirtatious charisma helps convince Othello that Cassio is having sex with Desdemona.
Cassio is the kind of guy who likes to put women in one of two categories – virgin or whore. When he talks about Desdemona, we can tell that he sees her as a kind of secular Virgin Mary. Here's what he says when Desdemona arrives in Cyprus:
O behold […] You men of Cyprus, let her have your knees,
Hail to thee lady, and the grace of heaven,
Before, behind thee, and on every hand,
Enwheel thee round. (2.1.91; 93-96)
Clearly, Cassio worships Desdemona but, he has a tendency to mock his courtesan girlfriend, Bianca, who, sadly, is pretty smitten with Cassio. As Iago points out, "when [Cassio] hears of [Bianca], he cannot refrain / From the excess of laughter" (4.1.115-116). While Cassio may not be guilty of sleeping with another man's wife, it seems pretty clear that he's kind of a jerk when it comes to women.