The associate of Julius Caesar involved in his assassination was Marcus Junius Brutus. Brutus was a prominent Roman senator and a close friend of Caesar, but he became a key conspirator in the plot to kill him on March 15, 44 BC, known as the Ides of March. Brutus and other senators feared that Caesar's increasing power threatened the Republic. Despite their friendship, Brutus chose to align himself with those opposing Caesar’s rule. His involvement has been historically romanticized and often depicted as a tragic betrayal, as he is known for his famous line, "Et tu, Brute?" in Shakespeare's play about Caesar's assassination.