Kono Jinruiiki no Zelphy

A former prince from a powerful dynasty, Reesha now lives as a common plumber in a hot springs town after his homeland was destroyed at the end of a war. For three years, he attended school with former subjects of his family, some of whom later joined the military. Due to his small stature caused by an artificial heart, Reesha has been repeatedly denied military entry. At 19, he and his military friends attempt to bypass the size requirement by sneaking into a bathhouse to plead their case to a general. Their plan leads to a series of violent incidents, ultimately forcing Reesha to leave behind his ordinary life and pursue an uncertain future.