At night, after battle, they lie in their tents and he writes her letters he'll never send; "if this reaches you, wait for me." He keeps them in his back pocket until they overflow and he is reminded of how her face shined on the battlefield, determined and loyal. She is extremely loyal to the army, to him, but it's never to a fault because he believes in his heart that she has none. During the day she is a solider, just as he is, and the words "start over again" hold no terror for them because they are ready to die at a moment's notice. It is their duty.
But at night duty no longer weighs on their shoulders and she sits outside and asks him how can disaster be so beautiful. He never answers because in truth he does not know the answer, only knows that there is something poetic in the way she speaks, in the curve of her chin and the tilt of her head. She is so delicate and Roy is reminded that there is much in this world to be saved from. His coat is caked with blood and he wonders if this is what it feels like to die.
He thinks about losing her and knows that fear is real and justified. It breaks his heart and he feels utterly and completely alone, but they aren't allowed to love because they are soldiers and he understands that. So he writes her letters and hopes that one day, after he is long gone, she will read them and remember him fondly. She will remark, "something with wings went wild in my chest once" when speaking of him and everyone there will know that he, Roy Mustang, was someone special in her life. He thinks it dramatic and naïve, but the thought never fails to bring a smile to his face.