Tokishige Usami is a superior private in the Seventh Division of the Imperial Japanese Army, a rank roughly equivalent to private first class, and he serves directly under First Lieutenant Tokushirou Tsurumi. He is a Japanese man of average height and muscular build, with pale skin, a youthful and somewhat androgynous face, and a pair of symmetrical moles on his cheeks. Usami grew up in a farming family in Niigata as one of several children, and as a boy he trained at Master Takeda's dojo alongside his schoolmate and close friend Tomoharu Takagi. From an early age he carried a strong sense of inferiority about his ordinary origins and a longing to be special. His childhood encounter with Tsurumi, who came from the same hometown, changed the course of his life: Tsurumi was the first person to recognize his judo talent and to acknowledge his worth as an individual. That experience became the foundation of Usami's identity, and he went on to become one of the earliest members of Tsurumi's circle within the Seventh Division, eventually serving in the Russo-Japanese War alongside Hyakunosuke Ogata.

Usami's personality is defined by intense aggression, unwavering loyalty, and a complete lack of guilt or remorse about killing. Tsurumi himself describes him as a born soldier, noting that while most soldiers are sheep, Usami is a dog among them. He is cheerful even in violent situations and takes genuine pleasure both in combat and in the attention, approval, and even punishment he receives from Tsurumi, displaying pronounced sadomasochistic tendencies. His loyalty, however, is not blind or thoughtless. He studies Tsurumi's behavior, understands the intentions and calculations behind his commands, and deliberately chooses to follow. A defining trait of his psychology is possessive jealousy: he cannot tolerate any rival for Tsurumi's regard and insists on being his number one person. This jealousy proved destructive in his youth, when Tsurumi remarked that Takagi was growing stronger than him. Unable to bear the thought of losing Tsurumi's approval, Usami killed his friend, and Tsurumi covered up the crime. That secret bound Usami to Tsurumi through a tangled mixture of guilt, gratitude, and worship that only deepened over time. When Tsurumi once drew stick figures over his facial moles with a fountain pen as punishment for a failure, Usami was so delighted that he had the drawings made into permanent tattoos.

In the story, Usami functions as one of Tsurumi's most trusted and effective soldiers during the pursuit of the hidden gold. He infiltrates Abashiri Prison disguised as a new guard, but he is discovered by the prison official Kadokura and forced to flee. He takes part in several combat operations, including a nighttime engagement in Hell Valley in which he impales an enemy with his bayonet while asking his fellow soldier Kikuta not to shoot. His martial competence is considerable; despite his relatively junior rank, his ability as a soldier is acknowledged even by Niusan, one of the strongest fighters in the series. His fierce devotion to Tsurumi makes him more aggressive and more dangerous than the average soldier, and he readily carries out killings sanctioned by his commander.

His most important relationship is with Tsurumi, who is the absolute center of his life and the source of his sense of self-worth. Toward most other people he is volatile and menacing, though he operates alongside fellow Seventh Division members such as Kikuta and Nikaidou. His relationship with Ogata is one of deep mutual hostility; he regards the traitorous sniper as his natural enemy, and Ogata ultimately becomes his killer. While in Sapporo investigating the murders of prostitutes, Usami encounters Ogata and gains the upper hand in their fight, but Ogata shoots him in the abdomen with a surprise attack. Usami manages to escape the immediate scene, only to be shot again in the chest at long range as he tries to flee on horseback, and this second wound proves fatal. As he lies dying, he happens to fall near Tsurumi, who recognizes that he cannot be saved. Tsurumi bites off Usami's little finger, mirroring the way he preserved the finger bones of his deceased wife and daughter, and tells him that he will live on in Tsurumi's heart as his number one friend. Usami's final words express that he is so happy he can hardly bear it, and he dies in bliss at receiving the recognition he had craved his entire life.

Usami's development across the story is less a change in character than a gradual revelation and completion of his devotion. His backstory explains how an ordinary boy's need for recognition was shaped into a fanatical and self-destructive loyalty, and his death provides a final resolution to that devotion, as he achieves the very thing he always wanted: to be Tsurumi's number one person. His notable abilities lie mainly in combat, including bayonet fighting, rifle use, and the judo skills he had trained since childhood, all driven by a fearlessness and eagerness for violence that mark him as one of the most aggressive soldiers in the Seventh Division.