Cassius is the architect of the conspiracy against Caesar: the Roman who persuades Brutus to join it, wins Casca to the group, and arranges the plan. Caesar describes him as lean and hungry, a man who reads a great deal, thinks too much, and takes no pleasure in music, and for that reason is dangerous.
In Act I, scene 2, alone with Brutus, he says that Brutus does not see his own face and offers himself as a mirror. He recounts that Caesar, on a windy day, challenged him to swim the Tiber and cried for help in the middle of the current, and that he saw him tremble with fever in Spain. He concludes that Rome bows before a weak man, and he leaves the scene resolved to send letters that look as if they came from citizens, to push Brutus. In the stormy night (scene 3), he walks bare-chested in the lightning and says that whoever wants to free himself from tyranny can always give up his own life.
In Act II, scene 1, he wants to kill Antony along with Caesar, and Brutus refuses. In Act III, scene 1, he warns that letting Antony speak at the funeral is a mistake, and Brutus defeats him again. At Sardis (Act IV, scene 3), he quarrels with Brutus over bribery, offers his breast to the dagger, and reconciles.
An Epicurean, he says at Philippi (Act V, scene 1), on his own birthday, that he has come to give partial credit to omens. In scene 3, he wrongly judges that Titinius has been captured and orders Pindarus to kill him with the sword that pierced Caesar. Brutus calls him the last of the Romans.