Who was Martha, and what did Jimmy Cross wonder about her?

In the eponymous story "The Things They Carried," Martha
is a girl in New Jersey who plays volleyball, goes to college, reads Virginia Woolf, and
writes Lt. Jimmy Cross letters.  Cross wonders if she is still a virgin.  In this way,
Cross seems like some Knight on a crusade, and he expects his maiden to wait faithfully
back at the castle.  It's an unrealistic fantasy, but Martha gives Cross hope.  Even
though she doesn't love him and he only loves the idea of her, she serves as a willing
audience, like the reader, who waits in judgment back home to decide if these men are
worthy.

Lt. Cross burns Martha’s letters and photographs
after one of his men die.  Cross (initials J.C. for "Jesus Christ") is a Christ-figure,
one who takes on the sins of all the others: namely guilt.  He feels responsible for
Lavender's death, blaming himself for thinking of Martha rather than his
men.

Martha never appears in the story, but she (and the
other American civilian women in the novel) represents home, warmth, protection, the
earth-mother.  The men in Alpha company dream of them and, through their letters, they
try to make these women understand war.  The women write back with only trivialities
about their daily lives, so there is a disconnect between Martha and Cross.  She will
never understand "what men do."  In fact, Cross says he loves her, but she cannot love
any male who has killed, raped, burned villages.

Later
Cross tells O'Brien, “Make me out to be a good guy, okay? Brave and handsome, all that
stuff. Best platoon leader ever. And do me a favor don’t mention anything about—“? What
isn’t O’Brien supposed to mention in his story about Cross?  Killing?  Rape?  Whatever
it is, O'Brien never tells us, or Martha.  She will never understand "the things men
do."