Issei Tsumuhana is the older brother of Isuzu Tsumuhana and a university student, approximately 24 years old. He possesses a tall, slim physique and is known for his clumsiness. Having grown up with Isuzu, he shares a close sibling bond. Upon Hiroshi Kuzumi and his sister Mana moving to Jōga Town, Issei presents as friendly and approachable, quickly befriending them. His reputation among the townspeople includes womanizing tendencies, often seen with multiple female companions, a habit his sister frequently teases him about.

Despite his amiable exterior, Issei conceals a darker nature tied to Jōga's secrets. As a resident, he belongs to the Kamibito, beings susceptible to predatory instincts triggered by the scent of humans, particularly potent "Temptations" like Hiroshi. Hiroshi's intoxicating scent proves difficult for Issei to resist, leading him to gradually succumb to his instincts. This results in increasingly erratic and aggressive behavior, culminating in an attempt to assault Hiroshi driven by uncontrollable urge, during which his eyes glow red.

Violating the strict rules governing the Kamibito results in his execution by Nemuru Kushinada, the town's Hunter tasked with eliminating threats to humans. During this execution in the anime adaptation, Issei reacts with hysterical laughter; in the visual novel, he faces death with defiance, locking eyes with Nemuru. His death profoundly impacts his sister Isuzu, contributing to her emotional decline and bitterness.

Issei displays ambiguous attraction toward Hiroshi, questioning Hiroshi's sexuality due to his reluctance to engage with Isuzu's advances. This culminates in Issei attempting forceful kisses and making inappropriate advances. He also openly flirts with Mana Kuzumi, complimenting her appearance shortly after meeting her, raising suspicions about his intentions. Additionally, he carries orange juice in his car, a detail speculated to be linked to the town's supernatural elements, though its exact significance remains unclear.