KAGE NO EIYUU NO NICHIJOU TAN: YUUSHA NO URA DE ANYAKU SHITEITA SAIKYOU NO AGENT. SOSHIKI GA KAITAI SARETA NO DE, SHOUTAI KAKUSHITE HITONAMI NO NICHIJOU WO OUKA SURU

The "Great War of Heroes", in which the hero and the demon king vie for survival and power in each incarnation. The fourth great war was also a war where the heroes defeated the Demon King...it's an undisclosed fact. Tweight, the ace of the “agency” in the dark side of kingdom, undertakes all the dirty work for country and supports the Hero to subjugation the Demon Lord from shadows. As the unsung hero, Tweight hopes to live a peaceful life after the war. However, defeating the Demon Lord is not enough to bring peace to the world. This is a story about an awkward boy who is a former “behind the scenes”. Fights, opposes, and tries as hard as he can to protect his daily life from him. — Description from AniList community (anilist.co).