The protagonist of the tragedy is Brutus - Although the play's title refers to Julius Caesar, Brutus's lines are four times more than Caesar's.
Mark Anthony is antagonist - He is antagonist because he wants to save his own life and in order to do this, he works with Brutus for the plot against Caesar.
Cassius is a foil - He knows Caesar for a longer time. He makes Brutus believe that Caesar became so powerful and therefore he must die.