major characters in 'the ides of march' – cytheris, lucius mamilius turrinus

There are two more characters of note in the 'fantasia' The Ides of March by Thornton Wilder.

Lucius Mamilius Turrinus is a fictional friend of Caesar, maimed in the Gallic Wars and living in seclusion on Capri.  Never appearing in person nor in writing, he is the dictator's confidant, and The Journal to him is the vehicle of Caesar's thoughts, which do drive the novel.  As posted before, the playwright Edward Sheldon (1886-1946) was the inspiration for the character of Turrinus.

The courtesan Cytheris has a brief mention in Smith, Dictionary of Greek and Roman Biography and Mythology:

CYTHERIS, a celebrated courtezan of the time of Cicero, Antony, and Gallus. She was originally the freedwoman and mistress of Volumnius Eutrapelus, and subsequently she became connected in the same capacity with Antony, and with Gallus the poet, to whom, however, she did not remain faithful.  Gallus mentioned her in his poems under the name of Lycoris, by which name she is spoken of also by the Scholiast Cruquius on Horace. (Sat. i. 2. 55, 10. 77 ; comp. Serv. ad Virg. Eclog. x. 1; Cic. Phil. ii. 24, ad Att. x. 10, 16, ad Fain. ix. 26 ; Plut. Ant. 9; Plin. //. N. viii. 16.) [L. S.]

In the novel, she is a likable and highly educated actress.*  Mistress of of Antonius as mentioned above, she is respected by the Dictator and a close friend of Turrinus.

List of all but very minor characters

The historian Cornelius Nepos mainly appears through his diary, his 'Common Place Book.'  Cicero's letters, I think, read much like the real thing.

*There are no female actors recorded in ancient Rome until the 6th century C.E., though there were mime-actresses acting at funerals.  They came by many names, among them Cytheris, as I discovered at JSTOR  (limited access):

A Republican Mime-Actress?
Charles Garton
The Classical Review > New Ser., Vol. 14, No. 3 (Dec., 1964), pp. 238-239