Answer:
The character that must decide whether or not to join the conspiracy is Brutus.
Step-by-step explanation:
Brutus is the most complex character from the play, he debates himself if what the conspirators are trying to do would be the best for the Republic. He is a loyal friend of Caesar but he is not in favor of the possibility of a dictatorship lead by him. He loves the Republic even more than his friends.
The rest of the conspirators with Cassius at the head of them convinced Brutus that the assassination of Caesar would be the best for everyone.
I hope this answer helps you.