Kono Jinruiiki no Zelphy

Reesha, once a prince of a powerful dynasty, is left without a home after a war ends. In the first three years of his new life, he attends school alongside former subjects who become his friends, but after graduation they all join the military. Reesha is rejected from military service three years in a row because his artificial heart has left him too small to meet the requirements. Now 19, he lives an ordinary life as a plumber in a hot-spring town. Hoping to get around the size requirement, Reesha and his military friends sneak into a bathhouse to appeal directly to a general. Their plan leads to unexpected violence, setting Reesha on a path away from everyday life as he pursues his destiny.