Who Marcus Brutus is
Marcus Brutus is trusted for his honesty and his love of the Roman republic, where no single man rules. His friendship with Caesar places that public loyalty beside a personal bond he does not easily dismiss.
What Marcus Brutus wants
He wants to protect Rome from kingship, although he has no personal quarrel with Caesar. His reasoning rests on what power might make Caesar become, rather than on a wrong Caesar has already done. That gap between fear and evidence makes his sense of duty troubling as well as sincere.
Marcus Brutus and the others in the story
Cassius needs the respect attached to Marcus Brutus's name and uses flattery to draw him into the plot. He also arranges forged letters that seem to ask for action from concerned Roman citizens.
Portia, Brutus's wife, sees his distress and asks him to share the secret he is keeping. Her appeal brings his private responsibilities into view while his thoughts turn toward Rome.
Questions
Is Marcus Brutus Caesar's friend?
Yes, Marcus Brutus is Caesar's friend, but his loyalty to the republic conflicts with that friendship.
Why does Marcus Brutus join the conspiracy?
He believes Caesar may become a king and threaten the republic. Cassius encourages that fear and appeals to his duty to Rome.
Is Marcus Brutus good or bad?
The play presents him as honorable but capable of dangerous reasoning. His sincere principles do not remove the danger of acting on fears about another man's future.