At the time of Nero, the Roman Empire was roughly 100 years old, having been started by Augustus Caesar. Gaius Julius Caesar declared himself Dictator for Life of the Roman Republic and was pretty famously stabbed to death on the floor of the Senate for it. Then his nephew (still going by Octavius at the time) fought a whole war against Marc Antony and Cleopatra over the right to rule Rome, changed his name to Augustus after he won, and changed Rome from a Republic to an Empire.
Romulus, being the mythical founder of Rome, was not an Emperor, but a King. There is a distinction that is important because there was almost 450 years of Republic between the Roman Monarchy and the Roman Empire.