Taro Sakamoto Nagumo and Sakamoto met when they were both enrolled in the TCG, starting at age 14. Nagumo has a deep affection for Sakamoto, to the point of not wanting him to be in danger, even though he knows his help is often needed. Nagumo has repeatedly demonstrated his close friendship with Sakamoto, staying in touch with him even after he retired from his assassin career and was hunted by the JAA. He has also done him favors and helped him, such as when he disguised himself as Aoi Sakamoto and posed as a cashier, killing Minimalist in the process.

Sakamoto and Nagumo maintained a close friendship over time and even collaborated, along with Kei Uzuki (who was dominated by Rion's personality), during their fight against Takamura at the Assassins of the Century Exhibition. Nagumo refused to follow Asaki's orders and chose to ally himself with Sakamoto, which led to both of their expulsion from the Order and their pursuit by the JAA.

Rion Akao Nagumo met Rion after being transferred from the Espionage Department to the Assassination Department, where he initially intended to attack him. Along with Sakamoto, they soon became an inseparable trio, causing property damage and engaging in pranks and fights. Their antics eventually led to their expulsion from the TCG, after which they met Kindaka and the Order together.

After Rion's death, Nagumo investigated it using various methods. This included paying for Kindaka's recovery expenses and rescuing Yotsumura from drowning so he could order her to search for information related to Rion. Nagumo has demonstrated his deep affection for Rion on numerous occasions, such as when he hesitated to attack Uzuki when the latter took control of his body and when he endured Takamura's attack to protect Uzuki.

Nagumo's most notable traits are his laid-back attitude, accompanied by a constant smile that seems impossible to wipe off his face. He also has an incredible talent for annoying those around him to the point of making them wish he were dead.

Nagumo also proves to be quite carefree, even in the midst of intense battles, once stating that he enjoyed the wind in his hair while stabbing a man in the arm. He is also very playful with his opponents, as demonstrated on several occasions. For example, he evades and tricks Minimalist, a hardened serial killer, before handing him dice to decide which of his swords he would die by. Another example is during his fight against Gaku, where he asked the latter if he liked Bruce Lee after receiving a one-inch punch.

However, when it comes to matters involving his friends, Nagumo tends to reveal his true emotions. An example of this is when he pulls Uzuki out of Takamura's cutting line, questioning why he saved him despite him being a dangerous serial killer, influenced by the fact that a part of Rion Akao still resided in Uzuki's body. In fact, Nagumo has shown deep affection for his old friends Sakamoto and Rion since his time in the JAA, still thinking of her even after her death was confirmed. This is further confirmed in their encounter during the Century Arc Assassin Exhibition, where, once Rion took control of Uzuki's body and spoke to him, Nagumo immediately recognized Rion's personality and questioned his identity.

Nagumo believes that the key to killing is "deception," despite claiming that he abhors it.

Despite this, Nagumo possesses a colder, more serious side. He acts in his own self-interest most of the time, and sometimes commits unethical acts. When he saved Satoru Yotsumura, he blackmailed him and threatened to kill Amane.

Nagumo has many tattoos, most of which are hidden by her clothing. Her tattoos have mathematical inspirations, in particular, they reference the Golden Ratio.

The tattoos shown include:

On the left side of his neck, he has a Fibonacci spiral mark. The growth factor of the Fibonacci spiral is the golden ratio, ~1.6180. Two rectangular tattoos on both hands, another reference to the golden ratio. The formula ϕ

1 + 1 + 1 + 1 + 1 + 1 + 1 + ⋯ on his right forearm. This is a presentation of an infinite sequence of nested square roots that resolves to ~1.6180... (the golden ratio). 5 rectangles with different designs (the third and fifth of them are black), next to the formula tattoo. The symbols, '2', ' ϕ {\displaystyle \phi}', '1' and '%' on his index, middle, ring and pinky fingers respectively. A snake coiled around his left leg. A swallow on the right side of her right leg, just below the knee. An impossible shape on his shin, beneath the swallow tattoo. A square formed by several spaced lines and curves on her right thigh. A mandala-like pattern on her right shoulder. A pair of hands in prayer on his left breast. Probably inspired by Albrecht Dürer's drawing of Hands in Prayer. The quote from Anton Chekhov: "Even if you tell lies, people will believe you, if you speak with authority" is tattooed on his right bicep. A line that crosses, wrapping around his front forearm. An isosceles triangle stamped on his left posterior forearm. A rectangular tattoo on his right tricep. A tattoo referencing Leonard Shelby from Christopher Nolan's Memento on his left bicep. A wing on her left waist. An unidentified tattoo under his left armpit

Nagumo is an attractive, tall, and slender man with medium-length black hair and large black eyes. He wears a red polo shirt with triangular patterns in teal, white, and black under a cream-colored trench coat (in the manga, the shirt is white with red and navy patterns, while in the digital version, the shirt is light blue and the coat is brown), navy blue pants (black in the digital version), and black shoes.

He occasionally wears a white button-down shirt with a black tie under a black suit (dark green in the manga) (sometimes omitting the suit and tie). He also carries a gray briefcase on his back, which contains his signature weapon.