He has notably rounded eyes. His overall features are quite youthful, and many comment that his smiling expression is cute. However, when facing an enemy or when extremely angry, his characteristic cold gaze becomes so chilling and eerie that one might doubt he is the same person. He is incredibly youthful-looking. His face hasn't changed at all since his JCC days eight years ago. In the story, a character who sees Nagumo for the first time in eight years is baffled by how he hasn't aged a day. Furthermore, when he first appeared, he lied about being 18, and Shin didn't even doubt him. He has many tattoos all over his body. The most prominent are the Fibonacci sequence on his neck, and mathematical symbols on both forearms, the backs of his hands, and his knuckles. Besides these, foreign quotes, various geometric shapes, birds, and snakes are tattooed on him. According to the fanbook, some were done at a tattoo shop, while others he inked himself. During his JCC days, he received 152 chocolates from female students on Valentine's Day, to the point he had to haul them in boxes on a cart. To put that in perspective, he received them from 152 out of 200 female students. He reportedly rejected chocolates from the Poison Division girls. He generally appears to have a very sly and playful personality, which he shows to allies and enemies alike. While he seems inscrutable, he is surprisingly one of the most deep-seatedly affectionate characters. His strong sense of camaraderie is often revealed, such as when he wears a lonely expression while mourning a fallen comrade, visits Sakamoto to warn him about a bounty, or tries to uncover the truth behind the death of Rion Akao, who was long thought dead. He cherishes his wife Guest and his son Yuichi 2,000 times more than that. He is a childish husband who is intensely jealous, even complaining that his own child has stolen his wife away from him.

The son of Nagumo Yoichi and Nagumo Guest. He is a total mommy's boy. Currently 7 months old, he bursts into tears whenever things don't go his way. He ignores his father but listens only to his mother. He can only babble for now.