The conspirators against Julius Caesar were concerned that Caesar was acting like a king, and there had been no kings in Rome for almost five hundred years. The last of the Roman kings, Tarquin, was, by tradition, expelled from the city in 509 BCE, and the man who played the leading role in this expulsion, and in the subsequent founding of the Roman Republic, was Lucius Junius Brutus.
Lucius Junius Brutus was by far the most famous among several illustrious ancestors of Marcus Junius Brutus. The gens Junia was one of the leading families in Rome and was, crucially, the one most associated with the Republic and its virtues. Before he conspired against his friend Caesar, Marcus Junius Brutus was more famous for his name and his ancestry than for anything he had actually done.
Brutus had, in fact, enjoyed a modestly successful, but not outstanding, career by the standards of a Roman statesman. He had fought on the side of the Optimates, against Caesar, in the civil war and had been pardoned for doing so. After this, he had held the rank of praetor in the Senate and then been Governor of Cisalpine Gaul. The idea that he had a particularly impressive reputation for personal integrity appears to be a modern fabrication (which appears prominently, for instance, in Shakespeare's Julius Caesar). His principal value to the conspiracy lay in his name.