Is Julius Caesar Caesar's tragedy?

Yes, not so much because of what happens to Caesar but more because of what happens to Brutus.

In the tragedy of Julius Caesar, Portia is the wife of Brutus, the idealist conspirator and main character of the story.

Fortuneteller

It is difficult to answer this one, because the play is not clearly structured in this way. However, the character closest to a protagonist must be Brutus. Antony is his antagonist.

Tragedy is one in which death of many important characters take place. The same was the case with the play "Julius Caesar" where the death of Julius Caesar himself, Brutus, Cassius, Portia, Cinna...