Eiji Kakihana is a forty-one-year-old man who works as a janitor, cleaning a building as his daily occupation. He is an old friend of the protagonists Hiroshi Odokawa and Ayumu Gouriki, with the three sharing a history that dates back to their high school days. Kakihana is characterized by his good-natured and optimistic personality, approaching life with a positive outlook that provides a contrast to the more cynical perspectives of those around him.

Despite his friendly demeanor, Kakihana struggles with feelings of loneliness and inadequacy, particularly regarding his age, modest income, and his status as a single man. These feelings drive his primary motivation throughout the story: a sincere and earnest desire to find a romantic partner and get married. In his pursuit of companionship, he actively uses a matching app, though his low self-esteem leads him to falsify his profile, claiming a much higher annual income and a university degree that he does not possess. This vulnerability makes him susceptible to manipulation, and he becomes entangled with a woman named Shiho Ichimura, who is part of a yakuza-backed honey trap scheme. Kakihana borrows money to maintain the facade of wealth, leading to his kidnapping and physical assault by yakuza members who mistakenly believe he has significant assets. Following his rescue, which is orchestrated by Odokawa, Kakihana finds himself burdened with a large debt, a consequence of the scam. He is forced to confront the fallout of his actions and pragmatically takes on multiple part-time jobs to steadily repay what he owes. His journey from naive optimism to a more tempered and realistic resilience is a key part of his character development, as he learns to face his problems independently.

Kakihana’s role in the story is that of a grounding, relatable figure whose everyday struggles highlight the series' more dramatic and dangerous events. His long-standing friendship with Odokawa and Gouriki provides a stable, personal connection for the main characters, and his personal saga of seeking love through an app explores themes of modern isolation and the gap between aspirations and reality. In terms of notable abilities, Kakihana possesses no special skills or combat prowess; his character is defined by his common humanity. Due to Odokawa’s visual agnosia, a psychological condition that causes him to perceive other humans as animals, Kakihana is initially seen as an anthropomorphic white gibbon, though he is ultimately revealed to be a human being.