Octavius Caesar: The adopted son of Caesar, Octavius by history, ultimately became ruler of the Roman Empire following his defeat of Mark Antony in Egypt (See Shakespeare's Antony and Cleopatra). In this play, Octavius with Mark Antony and Lepidus (The Second Triumvirate), destroy the forces of Brutus and Cassius on the Plains of Philippi, which results in the death of both these conspirators (Act V).
Unlike the other conspirators however, Cassius plays a leading role in Caesar's assassination. It is he who gathers those against Caesar around him and it is Cassius who carefully manipulates Brutus to their cause by appealing to Brutus' sense of civic duty which believes that Caesar as a King would be bad for the people of Rome and by Cassius' clever use of forged letters.
The Tragedy of Julius Caesar :|: Open Source Shakespeare
Caesar later returns in the play as a ghost which haunts Brutus in Act V. Easily flattered by Decius Brutus (not to be confused with Brutus), Caesar appears to us as a man almost guided not so much by his own will but what he believes are the expectations his people have of "Caesar." This is why he is reluctant to show fear, Caesar, as he frequently refers to himself in the third person, fears nothing and can show no sign of weakness or indeed mortality...
Julius Caesar, Act III, Scene 2 :|: Open Source Shakespeare
Flavius and Marullus: Two Tribunes introduced to us at the beginning of the play. Their conversation reveals the deep mistrust and fear many in Rome have about Caesar's growing popularity, which eventually leads to Caesar's assassination. These two men criticize Rome's citizens for praising Caesar almost without reason and are later put to death or "put to silence" for "pulling scarfs off Caesar's images," (Act I, Scene II, Line 291) during the Feast of Lupercal in Act I, Scene I (Note: Flavius the Tribune is not the same person as Flavius, a soldier whom appears in Act IV).