May These Leaden Battlegrounds Leave No Trace, Vol. 1 (light novel‪)‬

The power to rewrite history...and all it costs is a deal with the devil
For a hundred years, the East and West have waged a ferocious war using everything from tanks to witchcraft, but the technological advancements of the West have begun to shift the balance. That all changes one day when Rain Lantz, a young soldier from the East, finds a few mysterious silver bullets--and discovers that anyone shot with them is erased not only from the battlefield, but from history itself. Soon after, he meets the bullets' creator, a Ghost who calls herself Air, and learns that if he wishes to continue using this power to end the war for good, he will have to give her his freedom...