Marcus Brutus is the man in whom Rome trusts and of whom the conspiracy has need: a friend of Caesar, with no personal grievance against him, he ends up leading his death, believing that he acts for Rome.
He descends from the elder Brutus who expelled King Tarquin, is married to Portia, daughter of Cato, and is seduced by Cassius, who offers himself as the mirror that will show Brutus what he does not yet know about himself (Act I, scene 2). In the orchard (Act II, scene 1), he says he has no personal grievance against Caesar and decides on death for what he might become. He refuses the oath, spares Antony, and wants the act to look like a sacrifice, with sacrificers and not butchers.
In the Forum (Act III, scene 2), he explains the crime in prose and is acclaimed, then leaves the pulpit to Antony. At Sardis (Act IV, scene 3), he accuses Cassius of selling offices for gold, quarrels with him, reconciles, and reveals that Portia has died. At night, he sees Caesar's ghost, which promises to meet him at Philippi. At Philippi, Titinius says he gave the signal to attack too early.
In Act V, scene 1, Brutus says he finds it vile and cowardly to kill oneself out of fear of what may come, but that he will never be taken to Rome. In scene 5, he throws himself onto the sword that Strato holds, and Antony calls him the noblest of the Romans and says that only he, among the conspirators, acted for the common good.