Who was in overall control of the Roman empire?

it depends on the era but at one point it was julius casear and after him it was augustus caesar

It depends of the periods. During the monarchy (853-509 BC) it was the king, during the republic (509 BC-27 BC) it was two annualy elected consuls, during the principate (27 BC-285 BC) it was princeps and during the dominate (285-476 AD) it was the dominus.