In the final act of "Julius Caesar," who becomes the ruler of Rome?The end is Act 5, Scene 5. I'm unsure of whether or not there is a ruler, but...

As Shakespeare's play, "Julius Caesar" concludes, the reader perceives Octavius in a dominant role.  He has opposed Marc Antony's treacher to Lepidus earlier in Act IV, and now it is Octavius who speaks the final lines:

So call the field to rest, and let's away/To part the glories of this happy day. (V,v,80-82)

With the victory over Brutus and Cassius, along with the death of Brutus, the Second Triumvirate, Octavius, Antony, and Lepidus, go on to rule Rome.  However, Marc Antony who has been given the East, falls in love with Cleopatra and decides to fight Rome; Octavius, defeats Antony. With Antony out of the way and in control of the West and the East, Lepidus, who has been given Hispania and Africa, is forced to retire, and Octavius is free to become Augustus Caesar and become Emperor of Rome.  Of course, the irony of this situation is that Brutus killed Julius Caesar to prevent his becoming an emperor.  His and the other conspirators committed an act that only wrought negative changes:  Rome was engaged in civil strife, it was split for a time, and then subjected to the tyranny of an emperor, after all.