major characters in 'the ides of march' – julia marcia, cleopatra

Bust of  Cleopatra from the Altes Museum in Berlin, Germany.

Julia Marcia (Julia Caesaris), aunt of Julius Caesar and widow of Gaius Marius, is portrayed as the typical staunch Roman matron.  In real life long deceased, in the novel she is Caesar's contact to the Vestal Virgins with regard to the Bona Dea rites, this particular one, also anachronistically, a part of Wilder's 'fantasia.'

Smith, Dictionary of Greek and Roman Biography and Mythology has not much to say of her:

JULIA. 1. A daughter of C. Julius Caesar [caesar, No. 14] and Marcia, and aunt of Caesar the dictator. She married C. Marius the elder, by whom she had one son, C. Marius, slain at Praeneste in b. c. 82.  Julia died b. c. 68, and her nephew, C. Julius Caesar, pronounced her funeral oration, in which he traced her descent through the Marcii to Ancus, the fourth king of Rome, and through the Julii to Anchises and Venus.  At the funeral of Julia were exhibited, for the first time since Sulla's dictatorship in b. c. 81, the statues and inscriptive titles of the elder Marius. (Plut. Mar. 6, Goes. 1, 5 ; Suet. Goes. 6.)

Wikipedia (handle with care) adds some sources.

Cleopatra Queen of Egypt to me comes across very much as one would envision her from all the literature, highly intelligent in many ways, manipulative and somewhat stupid in others.  And contrary to some of the other characters in the novel, she was actually in Rome during the period in question, probably including the day of the assassination of Caesar.  In reality though, as a foreign ruler, she would not have been allowed to visit within Rome proper.

She gets a lengthy treatment in Smith, Dictionary of Greek and Roman Biography and Mythology.

Wkipedia has a decent page on her, nicely illustrated.