Cassius, described as "lean and hungry" in appearance, is a long-time acquaintance of Caesar who harbors envy toward Caesar's towering influence in Rome. Cassius is the catalyst in persuading Brutus to join the conspiracy against Caesar, demonstrating his willingness to use deceit by forging letters from the public to manipulate Brutus's opinion.

After Caesar's death, Cassius's character flaws come to the forefront when Brutus accuses him of miserliness and susceptibility to bribery, underscoring that Cassius's motives lack the nobility Brutus strives for. Cassius's cynicism, however, is balanced by his sharp insight. At three crucial points, he warns Brutus about Antony, advice that Brutus repeatedly dismisses, much to his detriment.

Though largely portrayed in a negative light, Cassius shows a redeeming loyalty to both Brutus and Titinius toward the play's end. This offers a glimpse of his character's complexity and reveals that his loyalty, though often compromised by ambition, is genuine when it matters most.