According to the Wikipedia entry on the book on which the movie is based, she is the maid, Michaela, and she represents innocence:
Toward the end of the novel, Aarfy rapes and murders a maid, Michaela,
while on leave in Rome. This inadvertently forms the emotional center
of the novel. When an aghast Yossarian tells him that he will be
arrested and possibly executed, Aarfy laughs dismissively that no one
would do that to "good old Aarfy":
"But I only raped her once!" he explained.
Yossarian was aghast. "But you killed her, Aarfy! You killed her!"
"Oh, I had to do that after I had raped her," Aarfy replied in his most condescending manner. "I couldn't very well let her go around
saying bad things about us, could I?"
"But why did you have to touch her at all, you dumb bastard?" Yossarian shouted. "Why couldn't you get yourself a girl off the
street if you wanted one? The city is full of prostitutes."
"Oh, no, not me," Aarfy bragged. "I never paid for it in my life."