Though Shakespeare's play is called Julius Caesar, the main character is Brutus. He is the play's tragic hero.
Aristotle described the characteristics of a tragic hero. He must be a great man—a man of notable deeds or character, or both. Second, the tragic hero has to die. Last, the hero's death is his own fault, brought on by his poor judgment because of a tragic flaw in his character.
Brutus is a great man in Rome. Brutus is greatly admired; when Rome erupts in Civil War, there are many that follow Brutus because of the true caliber of man he is. He is not a man who openly complains; while he worries over the fate of Rome in the hands of Caesar, he keeps his own counsel. Brutus believes in doing the honorable thing, even if it means his death:
For let the gods so speed me as I love
The name of honor more than I fear death. (I.ii.94-95)
Brutus tips his hand, letting Cassius know how he feels, especially in fearing that Caesar is to be crowned a king—in such a case, he says he'd rather be a peasant:
Brutus had rather be a villager
Than to repute himself a son of Rome
Under these hard conditions as this time
Is like to lay upon us. (178-181)
We know that Cassius lies to Brutus as he speaks of "seducing" the will of someone even as noble as Brutus—tricking him to believe there is more of a threat than there may really be—so Brutus will join Cassius. After Brutus leaves, Cassius ruminates about the man:
Well, Brutus, thou art noble; yet, I see
Thy honorable metal may be wrought
From that it is disposed; therefore it is meet
That noble minds keep ever with their likes;
For who so firm that cannot be seduced? (311-315)
Naiveté is Brutus' tragic flaw: he too easily believes Cassius—and falls in with Cassius' plot to kill Caesar. From one meeting to very next, Brutus has allowed himself to be swept into Cassius' conspiracy. Brutus will never know how this "friend" tricked him. At the end of the play, he erroneously honors Cassius:
My heart doth joy that yet in all my life
I found no man but he was true to me. (V.ii.38-39)
Brutus does die in this tragedy. He fights like a lion and is a valiant leader to his men. Rome is his only concern: he risks (and loses) his life for the good of his country. When he contemplates that he and Cassius may be defeated, he declares he will never allow himself to be captured and paraded through the streets in ignominy.
No, Cassius...Think not, thou noble Roman,
That ever Brutus will go bound to Rome;
He bears too great a mind. (V.i.120-122)
Brutus chooses a solider's death—to die by his own sword rather than be taken prisoner.