Cassius was the man who started the conspiracy against Caesar. He began the initial recruitment of Brutus and others and served as motivation for them, especially Brutus.
He main role, however, was to show just how noble Brutus was in comparison to the other conspirators. Even with Cassius's initial recruiting of Brutus, he established himself as a person jealous of the honors bestowed upon Caesar. He tries tempting Brutus be questioning what it was about Caesar that made him so powerful and Brutus and Cassius so ordinary.
Later in the play, we again see big differences between Cassius and Brutus. Cassius's armies are taking bribes to fund his battles, something that Brutus takes offense to (although Brutus also asks for money to help fund his armies!). Brutus's noble intentions are noticed by Anthony at the end of the play.
Cassius's death is meant with some basic grim irony. He, who was jealous of Caesar's attempts to become king, was crowned by Titanius after committing suicide. Not quite the way he wanted to obtain a crown for himself...