If Unbroken were a novel, Cynthia would be a character who came out of nowhere. She appears in the book's final part, and ends up being the love of Louie's life. He sees Cynthia at a bar shortly after returning home from the war. She has a "shimmer about her, an incandescence" (5.33.33), and when Louie sees her, he has the astounding thought that he has to marry her.
And he does.
It's not a happily ever after for either person, though. Cynthia is Louie's opposite, writing novels, painting, and yearning to see the world. Louie has already seen the world though, and while she's pretty well off, Louie is poor. Her parents don't want her to marry Louie, but the lovebirds do anyway on the sly.
Unfortunately, Cynthia doesn't understand Louie's drinking problem, which is getting worse every day do to the PTSD he suffers and his nightmares of the Bird. She's not quite sleeping with the enemy, like Julia Roberts, but "she was engaged to a stranger" (5.33.41). It's a major bummer.
Cynthia almost leaves Louie because he won't stop drinking, and she files for divorce after having a baby, little Cynthia, and coming home to find Louie shaking her. But it's thanks to Cynthia that Louie turns his life around. She convinces him to go see Billy Graham, the preacher, not once, but twice. If she hadn't done that, Louie might not have found the peace within himself, and the ability to forgive those who had wronged him during the war.