Yuusha, Yamemasu: Tsugi no Shokuba wa Maoujou

After saving the world by defeating the Demon Lord, the strongest hero Leo became unwanted in the peaceful human kingdom because they feared his power. Exiled, he sought a job with the Demon Lord's kingdom, which needs to be rebuilt after its defeat. The army and kingdom had many problems: lack of manpower, financial troubles, etc. Leo started to make improvements using his power and experience. When Leo encountered Echidna again, he asked her why she invaded the human world. There was an unexpected story---!
(Source: Kadokawa, translated)