This is a perplexing question to which there is no simple answer. In the same way that Shakespeare titled a play The Merchant of Venice, when the merchant, Antonio, is not the primary character in the play; in Julius Caesar, Caesar is not primary to the action that continues throughout the play. In both of these plays, however, the title character (in this case, Caesar) serves as the catalyst for action that is central to the play.
As for Brutus, the term "hero" can be misleading. We tend to think of a hero as someone who swoops in and saves the day, someone we would like to emulate. A super-hero is a good example of this sort of hero. However, if you consider the definition of tragic hero as it was described by the Greek philosopher Aristotle in his Poetics, then we have a very different definition of "hero."
As defined by Enotes, a tragic hero is:
the main character in a tragedy. The modern use of the term usually involves the notion that such an hero makes an error in his actions that leads to his or her downfall.. . .Aristotle. . . is quite clear in his pronouncement that the hero's misfortune is not brought about "by vice and depravity but by some error of judgment." In fact, in Aristotle's Poetics it is imperative that the tragic hero be noble.
And so, if we use the above definition, then the tragic hero is the main character, a noble man, who, through an error in judgement, causes his own downfall. Brutus does fit this description. He is in the play from first to last (many would argue that he is the play's main character) and is the author of his own downfall through his decision to join the Conspiracy against Caesar for "the good of Rome." This error in judgement causes his demise.
Caesar also causes his own downfall, in a way, by deciding to go to the Senate against all the warnings not to. However, he cannot be considered the main character in a play in which he dies in the Third Act. And yet, there is no doubt that the question "Who is the protagonist of Julius Caesar?" is one that continues to be debated about this play, and there are no cut and dried answers.
For more on tragic heroes, Brutus and Caesar, and which characters might be considered as a protagonist of this play, please follow the links below.
although the play Julius Caesar is known as Julius Caesar, the main character is Brutus, and Brutus can't really be called a "hero", yes he joined the conspiracy aqnd plotted against Caesar for the betterment of Rome, but he isn't really a hero for he has killed his friend and had been manipulated by Cassius, who had no intention regarding the betterment of Rome, but because Cassius had a personal reason ;
I was born free as Caesar, so were you;
We both have fed as well, and we can both
Endure the winter's cold as well as he.(105)
For once, upon a raw and gusty day,
The troubled Tiber chafing with her shores,
Caesar said to me, “Darest thou, Cassius, now
Leap in with me into this angry flood
And swim to yonder point?” Upon the word,(110)
Accoutred as I was, I plunged in
And bade him follow. So indeed he did.
The torrent roar'd, and we did buffet it
With lusty sinews, throwing it aside
And stemming it with hearts of controversy.(115)
But ere we could arrive the point proposed,
Caesar cried, “Help me, Cassius, or I sink!”
I, as Aeneas our great ancestor
Did from the flames of Troy upon his shoulder
The old Anchises bear, so from the waves of Tiber(120)
Did I the tired Caesar. And this man
Is now become a god, and Cassius is
A wretched creature, and must bend his body
If Caesar carelessly but nod on him.
He had a fever when he was in Spain,(125)
And when the fit was on him, I did mark
How he did shake. 'Tis true, this god did shake;
His coward lips did from their color fly,
And that same eye whose bend doth awe the world
Did lose his luster. I did hear him groan.(130)
while Brutus was focused on killing his dearest Caesar for the betterment of Rome, he thought that the conspirators had the same intention too, as we see from Cassius, the whole conspiracy was just selfish for they feared Caesar's overpowerment if he got the crown and thought more about themselves than about Rome. so Brutus being manipulated is not hero-ism.
and also we see in scene ii that Brutus had more internal conflict ; thus we conclude that the play "Julius Caesar" is all about Brutus and his emotions, thoughts, actions, etc.