Brutus - A more accurate title to the play would be Brutus. Of all the conspirators, as Marc Antony mentions in Act V, Brutus is the only conspirator who acted out of love for Rome. As the play opens, Cassius attempts to persuade Brutus to join the conspiracy through flattery and subterfuge. Brutus, despite being friends with Caesar, joins the conspirators, who realize Brutus' good name will bring them honor. Ironically, it is Brutus' poor decisions that doom them:
(1) Brutus argues in favor of Antony, that he not be killed with Caesar;
(2) Brutus allows Antony to speak at Caesar's funeral despite Cassisu' protestations;
(3) Brutus persuades Cassius to meet Antony's and Octavius' army at Philippi where the conspirators are finally defeated.
Cassius - Cassius was motivated primarily by jealousy. He is the true mastermind of the conspiracy. He flatters Brutus. He sends "anonymous" letters to Brutus that appear to be from Roman citizens. His shortcomings, however, doom the conspirators. He has neither the influence nor a good name and must defer to Brutus' wishes.
Casca - Casca appears early in the play and relates to Brutus and Cassius (and the audience) what transpires at the Feast of Lupercal in Act I. He is the first to stab Caesar.
Decius Brutus - His most important function is to flatter Caesar and makes sure he goes to the Capitol on March 15.
Trebonius, Ligarius, Metellus Cimber, Cinna - The remaining conspirators who take part in the assassination.