What is the function of minor characters such as Lucius,Titinius,Portia,and Artemidorus?William Shakespeare's Julius Caesar (other minor characters...

Minor characters in any literary work often serve as foils
to the main characters; that is, in the presentation of their characteristics, the lack
of these particular characteristics in the main characters becomes more apparent to the
readers/audience. In addition, minor characters' actions and words can help to advance
the plot or develop themes of a literary
work. 

• Portia, the
  wife of Brutus, is like Caesar's wife, Calpurnia, in that
  she wishes to share in the life of her husband and is concerned for his welfare.  Both
  Portia and Calpurnia are intuitive and sense the danger that surround their husbands. 
  Caplurnia's good sense points to Caesar's subverting his to his ego when he learns that
  he will be exalted if he goes to the Senate on the Ides of March.  And, if Brutus were
  to have told Portia his "counsels" as she begs, he may have not made his tragic mistake
  of slaying Caesar.  These women are foils to the
  men.


• Artemidorous is a
  friend of Caesar's who tries to warn him about Brutus and the other conspirators.  He
  stands in the street near the Capitol near the house of Brutus, hoping to give his
  letter to Caesar.  When he does place it in Caesar's hands, he tells Caesar it is a
  matter of personal reasons.  Because the letter is personal, Caesar fatefully postpones
  its reading.  The character of Artemidorous acts as a foil also, pointing to the unsound
  judgment of Caesar, thus, also, advancing the plot of
  assassination.


• Lucius is a
  servant to Brutus who delivers letters forged by Cassius highlighting Caesar's alleged
  ambitions. Clearly, he serves to advance the
  plot.


• Titinius is a friend
  of Brutus and Cassius.  In Act V he approaches nearby troops in order to learn if they
  are friends or enemies. When he is surrounded on his horse by cheers and
  cries, Cassius's bondsman, Pindarus, hears them and mistakenly interprets them as
  meaning that Titinius is captured. Fearing that he will also be captured, Cassius takes
  his own life. When Titinius returns and sees his friend slain, he mourns the end of Rome
  as he has known it and crowns the dead Cassius with a wreath, and then kills himself. 
  Titinius advances the plot and also points to the trope of the tragedy of
  miscommunication that prevails throughout the
  play.