The narrative begins with Kou, the hero, facing the Demon Lord, who was once his kind-hearted friend, Balrog. Kou is the only one left standing among his companions, including the knight Alexion, who is severely injured. The Demon Lord, in his monstrous form, attacks with magical projectiles. Kou uses his holy sword to deflect the attack, creating a diversion to close the distance. However, the Demon Lord strikes Kou's shoulder, causing immense pain. Despite his injury, Kou clings to the Demon Lord, calling out Balrog's name and expressing his regret for not being able to save him. He recalls Balrog's past as a kind mage who believed in magic's power to help people, and how human greed led to the death of someone Balrog loved, driving him to become the Demon Lord. Kou's emotional plea and determination to stop Balrog are met with a faint voice of Balrog, thanking him. Kou unleashes his final attack, bisecting the Demon Lord. Balrog's body crumbles to sand, and Kou, overwhelmed by grief and the weight of his mission, cries out in anguish.
Kou is then taken to the royal castle and summoned by the King. While his companions' injuries are healed, Kou remains wounded due to the hero's blessing interfering with healing magic. The King coldly declares Kou "unnecessary" and orders his execution, citing a prophecy that the hero and the Demon Lord are bound, and Kou's existence could lead to future Demon Lords. Alexion and Elena protest, but are restrained by their companion, Brad. Kou, resigned, closes his eyes as soldiers surround him, accepting his fate.
The story then abruptly shifts to a modern-day high school setting with a new protagonist, Kugi Kou. He wakes up late and rushes to school, contemplating skipping. He witnesses a beautiful girl being chased by a perverted old man and intervenes by kicking the man away. The girl, grateful, tries to thank him, but Kou, identifying himself as Ayase Kaito (a different person), deflects her gratitude and flees, believing he is merely a supporting character.
Kou's daily life as a "mob character" and best friend to the protagonist, Kaito, unfolds. He navigates school, dealing with Kaito's harem of heroines, including the tsundere Kiryu Kyoka and the energetic childhood friend Koto Tsumugi. Kou's interactions are often comedic and self-deprecating, highlighting his role as a side character. He also encounters Ayase Hikari, Kaito's younger sister, whom he had previously saved from the pervert. This encounter triggers a strong emotional reaction in Kou, reminiscent of his past trauma, causing him to flee.
Further events involve Kou's repeated encounters with the perverted old man, his ongoing struggles with schoolwork and detention, and his interactions with a peculiar first-year student named Yoshiki Yu. Kou finds himself drawn into Ayase Hikari's life, attempting to help her overcome her fear of men and her past trauma, while also grappling with his own unresolved issues and the potential complications of his relationship with her. The narrative explores Kou's internal conflict as he tries to maintain his role as a supporting character while dealing with increasingly significant events and relationships.
