Who was a key figure in the foundation of the Roman Republic?

Lucius Junius Brutus is recognized as a pivotal figure in the establishment of the Roman Republic in 509 BCE. His actions were central in the overthrow of the last Roman king, Tarquin the Proud, which marked the end of the monarchy and the beginning of the republican government system. Brutus's role did not just involve his leadership during the revolt but also included his subsequent efforts in creating the frameworks for the republic, such as the establishment of the consular office to govern Rome. He epitomizes the shift from autocratic rule to a government by elected officials, making his contribution foundational to the political structure that defined the Republic.

In contrast, figures like Julius Caesar and Augustus played significant roles in the later phases of Roman history — Caesar's actions ultimately led to the end of the Republic and the rise of imperial rule, while Augustus was the first emperor of Rome. Marcus Aurelius is known as a philosopher-king and a significant emperor in the later period of the Roman Empire, but he did not partake in the founding principles of the Republic. Thus, Lucius Junius Brutus stands out as the most critical individual directly associated with the origins of the Roman Republic.