This is an absolutely beautiful show, one that tugs at the heartstrings. Due to the underlying very German theme of the environment, and the incredulous voice acting, I recommend watching with German audio and your preferred subs. (I should also mention that this a Netflix original anime, so get your free trial ready.)
Synopsis:
A young, orphaned girl without a name is taken in by military officer Major Gilbert Bougainvillea, who, unlike his brother, treats the young girl as a human being, rather than a weapon for the war effort.
The girl, later named Violet by sir Gilbert, shows no sign of emotional understanding, and has shown to be hyperlethal in combat. After an attack on an enemy base goes wrong, Major Gilbert is presumed dead but labelled 'missing in action', as there is no body found. Violet, sure that the Major is still alive, goes on a quest to understand what his last words to her meant, the words, "I love you."