Yoichi Nagumo is a major supporting figure in Sakamoto Days, known for his easygoing yet unreadable demeanor. He comes from a long line of spies and was raised to inherit that dangerous legacy. At the age of fourteen, he enrolled in the Japan Clear Creation academy, an elite assassin training school, where he quickly distinguished himself alongside Taro Sakamoto and Rion Akao as one of the three greatest prospects the institution had ever seen. After graduating, he became a member of the Order, the elite assassination squad operating under the Japan Assassin Association. He initially worked as a classmate and colleague of Sakamoto until the latter retired to start a family.

Nagumo is tall and slim, with medium-length black hair and large black eyes. His most distinctive physical feature is the extensive collection of tattoos hidden beneath his clothes, many of which are inspired by mathematics and philosophy, including the Fibonacci spiral, the golden ratio, and quotations from thinkers such as Anton Chekhov and Arthur Schopenhauer. He typically dresses casually, favoring a red polo shirt with triangular patterns.

Personality-wise, Nagumo presents a cheerful, playful, and seemingly careless exterior. He enjoys lying, teasing others, and antagonizing his fellow Order members, making it nearly impossible for anyone to read his true motives. Beneath this guise, however, he is a cold, calculating, and highly perceptive individual. He is capable of making rapid and accurate deductions, such as instantly identifying a high-ranking JAA executive as an enemy spy. His intelligence and strategic thinking make him one of the most cunning members of the Order; he prefers deception and carefully laid traps over direct confrontation.

Nagumo's primary motivation appears to be a deep loyalty to his old friends, particularly Sakamoto. Despite his usual flippancy, he holds Sakamoto in high regard and goes to great lengths to protect the peaceful life his former colleague has built. He first re-enters Sakamoto's life to warn him about a bounty placed on his head, and later, after being framed by the Order's leader, Oki, he fully commits to siding with Sakamoto, even at the cost of being hunted by the entire JAA.

His abilities are formidable and multifaceted. He possesses superhuman strength, speed, reflexes, and durability, allowing him to hold his own in close-quarters combat. His martial arts are complemented by a trick-fighting style mixed with bluffing. He is also an extremely skilled marksman, ranking fourth among the top ten shooters in JCC history, even though firearms are not his primary weapon. His signature armament is a multipurpose tool outfitted with various interchangeable blades. His most remarkable skill, however, is his near-perfect ability to disguise himself. Through exceptional acting and body control, he can flawlessly mimic another person's voice, speech patterns, facial expressions, and entire appearance, achieving an effect that borders on shapeshifting. He requires no external tools or makeup to accomplish this.

In the broader story, Nagumo serves as a bridge between the world of assassins and Sakamoto's new, peaceful life. His relationship with Sakamoto is one of the most enduring in the series; the two have known each other since they were teenagers and share a bond of deep mutual respect. His connection to the late Rion Akao also ties him to larger, unresolved conflicts involving the Order and the JAA. Throughout the series, Nagumo develops from a seemingly aloof operative into a committed ally who willingly gives up his position and safety to stand beside his friends. His combination of a relaxed surface personality, lethal competence, and genuine loyalty has made him one of the most popular characters in the series.