There has been much debate over just who the main character of the play actually is. Opinion is divided as to whether it is Caesar, or Marcus Brutus, Caesar's one-time friend and the most conspicuous of his killers. Brutus certainly appears in many more scenes than Caesar; his moral dilemma over Caesar's killing absorbs most of the play's interest, and the play ends with his death. In his nobility of bearing (recognised at the end even by his enemy Mark Antony), his political idealism, and the moral trials he endures, he certainly appears to qualify as the tragic hero of the play.
On the other hand, Julius Caesar is the titular character, which reflects how the action of the play essentially revolves around him, even although he appears in person in only a very few scenes, and dies halfway through. His murder is the central event of the play; we see first the build-up, the forming of the conspiracy, and then the bloody aftermath as Rome descends into civil war.
Caesar's influence does not diminish after his death; rather, it increases, as his nephew Octavius, and Mark Antony do battle with the conspirators and emerge victorious. Ultimately the murder of Julius Caesar only leads to the establishment of another, even more powerful Caesar; Octavius will go on to become the first Emperor of Rome. This is the very reverse of what Brutus had hoped.
Therefore Julius Caesar can be regarded as the main character as, even in death, he continues to dominate the action. Brutus himself recognises this only too well:
O Julius Caesar, thou are mighty yet!
Thy spirit walks abroad, and turns our swords
In our own proper entrails. (V.iii.94-96)
And, when he finally falls on his own sword, he invokes the man he killed once more:
Caesar, now be still,
I killed not thee with half so good a will. (V.v.50-51)