Gin no Yuusha

From Storm in Heaven:

Beat has always dreamed of being the Golden Hero of Totia. He made a pact with his best friend Ricercare (Rice for short) to join forces to annihilate the demon king in order to achieve this aim. Yet a twist of fate causes him to abandon his goals and fight alongside Rice to protect the demons. Just how will they fare against the entire nation of Totia? And can Beat protect Rice from the hero of Totia before it's too late? Cute and heart-warming, this amusing five-volume series also includes interesting side stories rich with magic and beautiful art.