Brutus in Shakespeare's Julius Caesar is the main character, the protagonist, the tragic figure.
The conspirators will not go ahead with assassinating Caesar without Brutus's approval and participation. He alone carries the political weight to pull off the assassination. He chooses to join the conspiracy, and takes over the leadership of the conspiracy.
His leadership, however, not only leads to Caesar's assassination, but to the chaos that follows Caesar's death and to the failure of the conspiracy. His leadership also leads to his own fall from glory and death.
Brutus makes poor decisions. Ironically, the less noble Cassius turns out to be a much better decision maker than Brutus. Cassius wants to kill Antony along with Caesar, but Brutus says no. Cassius doesn't want to let Antony speak at Caesar's funeral, but Brutus vetoes him. Cassius wants to maintain and hold a strong defensive position in the play's final battle, but Brutus orders the armies to attack with an offensive strategy. These decisions lead to the failure of the conspiracy and to the deaths of the conspirators, including Brutus.
Brutus is noble, but idealistic and possibly full of self-deception. He is the focus of the play, however, and his decisions and actions create the tragedy in the drama.