The protagonist of Shakespeare's Julius Caesar is probably Brutus, "the noblest Roman of them all." He is a typical tragic protagonist in that he is a genuinely admirable man devoted to the ideals of the Republic and acting out of ethical beliefs rather than self interest. He has, however, several flaws that contribute to the play being a tragedy.
His first major flaw is that he is too trusting and thus does not understand the degree to which Cassius and the other assassins are motivated by jealousy and self-interest. He also, despite his good motives, participates in murder of Caesar, a man who has always been a good friend to him, and thus his rigid sense of political ethics leads him to an evil act.
There are really several antagonists in the play, but perhaps the most important one is Mark Antony, a manipulative demagogue who, like Caesar, caters to the mob.
While some people have suggested Caesar as a protagonist, he dies in the third act and does not really develop as a character.
There are many disagreements about this issue, actually, and a case can be made for several different answers.
Here are some of these scenarios and rationales:
- Julius Caesar is the protagonist, or hero. This is because the play is named after him, and he is the leader of Rome during the play. If Caesar is the hero, then the antagonists become the group of conspirators who plot to kill him.
- Brutus is the hero. He is the one who truly loves Rome, and acts honorably out of a sincere desire to do what is good for Rome. If Brutus is the hero, then the antagonist can be Cassius or Caesar, because at different times in the play both work against Brutus.
- Antony is the hero. He is the one who acted honorably thorughout the play, making positive choices and not trying to kill anyone. He is loyal to his friend, Caesar, and after Caesar's death he tries to take down the conspirators who threatened the Roman government.
You can decide for yourself which of these scenarios you think is the most valid, and support that one.