Kengo Asamura is a sixteen-year-old classmate and childhood friend of the protagonist Akira Nikaido. He has a lean but durable build, standing 173 centimeters tall, with naturally black hair that he bleaches blond. His brown eyes and cheerful expression give him an approachable look, and he typically wears his school uniform along with red earrings, a stud in his left ear and a stud plus a hoop in his right.

Kengo is defined by an exceptionally optimistic and loyal personality. He is highly tolerant and patient, rarely holding a grudge even when subjected to harsh words or physical treatment from Akira and others. Although often called an idiot by those around him, he possesses average intelligence and is simply straightforward in his thinking. His loyalty is his most defining trait, as he follows Akira with an almost dogged devotion, willing to endure any hardship to stay by his side. Beneath his cheerful and sometimes childish exterior, however, lies a deep-seated insecurity stemming from a childhood in which other children avoided him. He uses his upbeat demeanor to mask any hurt and is determined not to be a burden to his friends, striving constantly to prove himself useful.

His primary motivation is his desire to protect those he cares about, particularly Akira, and to be a reliable ally rather than a liability. This drive pushes him to join the fight against the shadowy forces known as Kokuchi, despite initially lacking any special powers. His role in the story is that of the steadfast human companion who provides emotional support and physical backup in battle. He acts as a bridge between the ordinary world and the supernatural conflicts Akira becomes involved in, often serving as comic relief due to his naive questions but proving his courage and resolve when it matters most.

The most significant relationship in Kengo's life is his bond with Akira Nikaido. They met when Akira transferred to his elementary school, and Kengo persistently sought out the aloof new student, eventually forging a deep friendship that lasts for years. Despite Akira's often dismissive and abusive treatment, Kengo remains unfailingly loyal, viewing his role as Akira's trusted companion as his natural place. He also has a caring relationship with his older sister, Mayu Asamura, and his desire to rescue her from a Kokuchi possession was a catalyst for his latent abilities to first surface.

Throughout the series, Kengo undergoes significant development, transitioning from a seemingly ordinary, if somewhat rowdy, teenager into a capable fighter. His hidden nature as a holder of a dark factor is gradually revealed. This factor makes him highly susceptible to absorbing dark energy, granting him an exceptional tolerance for the shadow realm but also causing him to enter an uncontrollable berserker state when he absorbs too much darkness, during which his physical strength multiplies dramatically. Kō, recognizing this dangerous potential, sealed his ability to absorb darkness early in their acquaintance, a seal that was later broken by the antagonist Homurabi. He receives a pair of gloves called Arisista from Shirogane, which transform into powerful gauntlets that enhance his physical attacks in combat. He develops his own boxing-based fighting style, proving to be a formidable hand-to-hand combatant through his endurance and hard-hitting power rather than technique or agility. In the final battle of the manga, his dark factor is forcibly removed by Homurabi, leading to his death. He passes away with a smile while looking at Akira, having willingly sacrificed himself for his friends.