Spoilers for up to chapter 61 of the manga. Disclaimer: I don't own FMA.
A Step Behind
She always did tread a step behind him.
Some thought it was symbolic of her servitude—a lower-ranking soldier falling behind her commanding officer as a show of respect. Others believed it was so she could always guard his back from the enemy, because, after all, military officers did make quite a few for themselves.
Those who thought such things were right, of course; simplifying the relationship between officer and subordinate so that it could be easily explained, easily understood. Pretty, noble words that any man, woman, or child could digest and comprehend.
The truth—the real, full truth—however, was slightly more complex than all of that.
She stood behind him, following him with carefully measured steps, because it was her job. Not simply as a soldier or as an aide, but as a person who had bared her soul to him, and had been given only the same in return. They had spied one another briefly through open doors and windows as children—been given fleeting glimpses of the person with whom they would spend the rest of their lives. Together, they had stood side-by-side before her father's grave, paying respects and wondering when everything had gone wrong. In the dark hush of night, with her spread across the desk in his dormitory, he had hunched over her naked back and ran his callused fingers over her scarred skin. She understood him more completely than anyone ever had before, simply because he told her everything that he thought, felt, dreamt, or wondered about. And she, in return, did the same for him.
She was the secret behind his alchemy. She was the initial spark that ignited the streams of fire he shot from his fingertips. For so many reasons, she was his very most prized possession.
And so, she stood behind him. Not necessarily out of duty, but because he had asked her to, once, long ago when she had first been assigned beneath him. To guard him. To watch the path over which he walked, and to make sure he didn't stray.
Because there was her pistol against his spine, and she was ready to pull the trigger if ever he did.