Re:ZERO Ex, Vol. 2: The Love Song of the Sword Devil (light novel) — Tappei Nagatsuki

When a devil falls for an angel...Long ago, a great war raged between the royal military of Lugunica and the demi-humans. On that battlefield, a noteworthy young warrior was born---Wilhelm Trias, dubbed the Sword Devil. But despite his skill with a blade, the boy would need to grow much more before he could become a true hero. This is the story of a legendary swordsman--and how he learned to love.