Aki Hayakawa's peaceful childhood in Hokkaido, Japan, ended when the Gun Devil slaughtered his parents and younger brother, Taiyo, while he was outside. This trauma drove his quest for vengeance, leading him to join the Public Safety Devil Hunters at sixteen, solely dedicated to killing the Gun Devil. He cultivated a stoic, reserved demeanor, presenting disciplined emotional control. Strict routines like morning coffee and reading, alongside a neat appearance, masked deeper struggles; his rigid self-discipline countered feelings of powerlessness in a devil-ravaged world. Eschewing hedonism like some colleagues, he channeled grief into perfectionism and restraint. Yet colleagues like Himeno sensed his underlying "madness," particularly his suicidal insistence on defeating the Gun Devil alone—a pursuit stemming less from hatred than from an existential void; without vengeance, he faced overwhelming emptiness. To enhance his combat abilities, Aki contracted multiple devils at great cost: - The Fox Devil required skin offerings to summon its head for devouring enemies, a contract lost after reckless use against the Katana Man. - The Curse Devil empowered a sword to kill targets upon three strikes but drastically shortened his lifespan per use. - The Future Devil resided in his right eye, granting seconds of precognition in exchange for enduring visions of his "worst possible death." - Unwittingly, he entered a fourth contract with Makima (the Control Devil), enabling his possession by the Gun Devil. Beyond contracts, Aki possessed exceptional swordsmanship, reflexes, and durability, surviving injuries like stabbings and crushing force. His relationships evolved significantly. Initially despising all devils as tools or enemies, he clashed violently with Denji over perceived frivolity and Power over her chaotic behavior. Cohabitation and shared missions fostered familial bonds. He cooked for them, protected them from threats like Kobeni, and prioritized their safety over revenge, even requesting division reassignment to shield them. His partnership with Himeno deepened into mutual trust; she sparked his smoking habit—a motif symbolizing their connection. Her death devastated him; cigarettes later triggered grief but also resolve during battles, like when the Ghost Devil (influenced by Himeno's will) handed him a cigarette, reigniting his will to fight. He also softened toward the Angel Devil, sacrificing lifespan to save him and writing a recommendation to spare his life. Unrequited feelings for Makima complicated his loyalty; her manipulation exploited his trust, and he could not articulate why he admired her, implying her control powers influenced him. Critical events reshaped his motivations. After the Future Devil revealed visions of Denji killing him and Power, Aki prioritized protecting his newfound family over vengeance. He pleaded with Makima to ensure their survival, unaware of her betrayal. This plea culminated in her forcing him into the contract enabling the Gun Devil's possession, transforming him into the Gun Fiend. Mutated, a pistol barrel protruded from his forehead, his left arm became a rifle, and dark veins covered his eyes. Forced to attack civilians and friends, he perceived his final battle with Denji as a snowball fight—a distorted echo of childhood innocence. Denji, compelled to kill him, ended his life. In death, Aki envisioned reuniting with Taiyo. Symbolism permeated his character: his name "Aki" referenced the AK-47, foreshadowing his fusion with the Gun Devil, while "Hayakawa" (meaning "swift river") hinted at his truncated life. His topknot hairstyle later mirrored the gun hammer in his fiend form. His will allocated half his savings to Denji and Power and half to Himeno's family.