Kouki Amanogawa is a Japanese high school student renowned for exceptional academics, athletics, and kendo, earning him a "perfect superman" reputation. His worldview, shaped by his grandfather's idealized hero tales, equates absolute justice with majority opinion. Lacking exposure to failure or moral complexity, he developed an unyielding, self-righteous mindset. His family consists of a consultant father, a former delinquent mother working as a magazine editor, and a younger sister who admires his childhood friend Shizuku Yaegashi. Kouki trained alongside Shizuku at the Yaegashi Dojo, achieving national-level kendo proficiency.

Summoned to the fantasy world Tortus with his class, Kouki received the "Hero" job class, granting superior physical stats, immense mana, and proficiency in all elemental magics, especially light-based spells amplified by his Holy Sword artifact. His combat abilities include enhanced strength, speed, durability, and sensory skills like "Sense Presence" and "Sense Magic." He blends Yaegashi-style swordsmanship techniques—"Pale Moon" and "Mist Piercer"—with Heiligh Kingdom Knight Sword Arts like "Rotation Blade." Later, he develops unique skills such as "Flowing Blade Reversal" and "Limit Break" for temporary power surges. However, hesitation in lethal situations and a tendency towards impractical, flashy attacks undermine his effectiveness.

Personality-wise, Kouki initially appears charismatic and justice-driven, but beneath this lies deep-seated arrogance, hypocrisy, and naivete. He enforces rigid black-and-white morality, refusing to kill enemies even when necessary and condemning others like Hajime Nagumo for doing so. His leadership suffers from poor decision-making, endangering allies through reluctance to adapt tactics. He harbors possessiveness towards childhood friends Kaori Shirasaki and Shizuku Yaegashi, mistaking camaraderie for romantic love and reacting with jealousy when Kaori pursues Hajime. This culminates in multiple delusional challenges against Hajime, accusing him of "brainwashing" female companions; each ends in humiliating defeat, exposing Kouki's inferior combat skills and emotional fragility.

Kouki's character deteriorates under Tortus's harsh realities. Failing to acquire "Age of Gods" magic during labyrinth trials—unlike his peers—leaves him the weakest among advanced fighters. Manipulated by Eri Nakamura's brainwashing and Ehit's influence, he betrays his class during the Schnee Labyrinth arc, attempting to kill Hajime and seize his companions to "save the world." This act results in his abandonment by Ehit and near-fatal injuries from Hajime. Post-brainwashing, he sinks into guilt-induced depression and faces further ostracism for aiding the antagonist.

Redemption begins in the After Story as Kouki returns to Tortus to atone. Initially wandering as a reckless adventurer, he flinches at the "hero" title and isolates himself. Summoned to the Desert World, he confronts Moana Synclea's plight against "Dark Beings." A near-fatal injury and a child's plea force him to reconcile his ideals with war's brutality, acknowledging past failures like his grandfather's flawed teachings, Shizuku's criticisms, and Eri's obsession. During a solitary last stand against thousands of Dark Beings, he sheds his delusions, demonstrating genuine selflessness and resolve. His actions save countless lives, earning Hajime's reluctant respect and assistance in defending Synclea Kingdom. This arc concludes with Kouki embracing a grounded, protective role, free from his former savior complex.