Youmu Farmenas began as a human criminal imprisoned in Falmuth. Count Nidol Migam placed him in charge of the disposable Frontier Expedition Force, scouting the Orc Lord in Jura Forest. There, Youmu asserted leadership by intimidating Rommel into breaking the group's contract magic, securing Rommel's loyalty. His path shifted upon meeting Rimuru Tempest in the monster settlement of Tempest. Seeking human-monster reconciliation, Rimuru chose Youmu to publicly take credit for defeating the Orc Lord. Witnessing Tempest's harmony, the hesitant Youmu agreed.

Embracing the role of a wandering hero, Youmu underwent intense training under Hakurou, elevating his combat prowess to advanced demon levels. Armed with specialized gear like the Dragon Slayer greatsword and lightweight Exo Armor forged from armored spider exoskeletons, he formed the hero squad "Youm the Hero and His Squad." Clad in eye-catching coordinated armor, they traveled widely, defeating monsters and building renown. During his journeys, Youmu recruited Grucius from the Beast Kingdom, forging a bond of friendship and rivalry. He also encountered Mjurran, who infiltrated his party under orders from Demon Lord Clayman. Despite her initial deception, genuine romantic feelings developed between them.

Youmu faced severe trials during Falmuth's assault on Tempest. Mjurran, under Clayman's control, participated in the attack. Youmu nearly died shielding her but was saved by Rimuru's intervention. Following Rimuru's evolution into a True Demon Lord, Mjurran was freed from Clayman. Heeding Rimuru's counsel, Youmu then worked to establish a human kingdom friendly to monsters. With strategic aid from Rimuru and Diablo, he orchestrated King Edward Falmuth's removal. Ascending the throne, he renamed the nation the Kingdom of Farmenas. His reign prioritizes bridging human-monster relations and engaging in international diplomacy.

Physically, Youmu possesses a powerfully built, sun-bronzed physique and average height. His bearing reflects constant vigilance, featuring a handsome face that belies his rugged, delinquent past. As king, he exudes regal authority. Personality-wise, he displays innate charisma and leadership, uniting diverse followers through loyalty and trust. While favoring casual interactions privately, he publicly embodies the heroic ideal. His actions are driven by a sense of justice, empathy, and protective instincts, valuing authentic connections over status. His leadership fosters unity, inspiring his team and nation to champion cooperation between species.