Jin Kōgasaki attends Rasetsu Academy, immediately recognizable by his light-colored hair, slanted eyes, and standard uniform always paired with a distinctive face mask. This mask hides numerous stitched scars covering his body, particularly the lower jaw, remnants of childhood trauma inflicted by his father, Hayate Momoid. Despite a handsome face, these prominent scars drive his need for concealment. He stands 172 cm tall, with blond hair featuring brown undertones and an ahoge.

Born Jin Momoid, he enjoyed a loving family, deeply admiring his father Hayate and sharing warm bonds with his mother and older sister. This life shattered when Hayate, discovering their oni heritage, murdered Jin's mother and sister. Jin survived the attack but sustained severe injuries, causing his scars. Forced into solitude, he endured harsh conditions, scavenging scraps and sheltering in ventilation ducts before enrolling at Rasetsu Academy to pursue vengeance against his father.

His personality presents a calm, analytical exterior masking intense volatility. He rationally assesses situations but harbors a fierce temper, especially toward obstacles blocking his goals. Initially uncooperative and dismissive of teamwork, he prioritized personal vengeance, attacking allies indiscriminately during early assignments. Despite his lone-wolf demeanor and blunt speech, he shows underlying kindness toward vulnerable individuals like endangered children. Emotional vulnerability surfaces around themes of familial loss, revealing a sentimental core beneath his icy facade. Interactions with peers, particularly Shiki Ichinose, gradually foster mutual trust and cooperative instincts.

Jin wields "Blood Corruption Release," manipulating his blood into weaponized saws and chainsaws by opening his body's stitches. His techniques include:
- "Blood of the Seven Sanctions": Summoning saws from multiple stitched points.
- "Dance of the Severing Blood": Launching chainsaw-shaped blood projectiles.
- "Severed Blood Blizzard": Releasing high-speed micro-saws for wide-area coverage.
- "Draw-Sever": Employing compact hand-mounted saws for close combat.
- "Severing Blood Barrage": Rapid-firing saw projectiles.
His combat style integrates enhanced mobility, deploying saws on his feet for rapid traversal.

Relationships evolve significantly. His dynamic with Shiki Ichinose begins antagonistically, marked by clashes and friction. Shared trials—surviving the Nerima Arc or coordinating Kegon Falls evacuations—foster deepening camaraderie and mutual reliance. Initially distant toward classmates like Homare Byobugaura and Ikari Yaoroshi, Jin gradually assumes leadership, mentoring peers during Snow Mountain Training and orchestrating team rescues. His bond with instructor Naito Mudano transitions from defiance to respectful adherence after Mudano curbs his self-destructive tendencies.

Jin's story arcs from isolation to integration. Early portrayals show a vengeful outlier rejecting alliances during initial academy challenges. The Nerima Arc marks a turning point: after abduction and manipulation by Shin’ya’s faction, he collaborates with Shiki to save civilians from a hospital fire, cementing their trust. During Snow Mountain Training, he embraces teamwork, guiding struggling peers, winning the summit race, and reaffirming his resolve against Hayate. In the Kegon Falls Laboratory Arc, he commands student forces strategically, overcoming adversaries like Melody and leading evacuation efforts amid collapse. Later, in the Koenji Arc, he proactively supports Shiki during psychological turmoil and helps neutralize betrayals, prioritizing collective safety over personal vendettas.

Trivia notes include his birthday (September 17), Virgo zodiac sign, and a concealed preference for sweets. The character was conceived to contrast Shiki Ichinose, emphasizing Jin's journey from solitary rage to tempered solidarity.