Ayumu Gouriki directs the Gouriki Medical Clinic and is the primary physician for Hiroshi Odokawa. He balances professionalism with genuine concern for Odokawa's well-being, reflecting their long-standing patient-doctor relationship and underlying friendship. Gouriki possesses a calm, intelligent, and serious demeanor. He has a wife and children.

His connection with Odokawa traces back to at least 2018, beginning with a documented taxi ride encounter that established their ongoing professional and personal bond. Throughout the series, Gouriki remains vigilant about Odokawa's health, particularly regarding his unexplained medical condition and reclusive nature.

A final episode revelation clarifies that Gouriki's perceived anthropomorphic gorilla appearance stems from Odokawa's visual agnosia, a neurological condition causing distorted perception. Gouriki is human, consistent with all other characters. This twist recontextualizes earlier interactions while maintaining his role as a dedicated medical professional.

Gouriki experiences no significant character development or expanded background across the series, film, or supplementary media. His narrative function centers on providing medical oversight for Odokawa and offering insights into the protagonist's psychological state. He serves as a consistent stabilizing presence amidst the interconnected mysteries affecting others.