Junichi Sakamoto is a character who appears in the later narrative arcs of Initial D, making his debut during the team's expansion into new territories. As a member of the Spiral ZERO team, which operates on the Nanamagari pass, he holds the position of the team's second driver. He is known within the team by the moniker Zero Two, a designation that reflects his close alignment with the team's core philosophy. Physically, he is depicted as an adult male with black hair and black eyes. While his given name is cited in supplementary material as Junichi, the narrative itself most often refers to him solely by his surname, Sakamoto.

Sakamoto's most defining characteristic is his profound and unwavering belief in the Zero Theory, a driving philosophy developed by his team leader, Ryuji Ikeda. This theory emphasizes a state of mental clarity and emotional detachment behind the wheel, positing that a driver must eliminate all subjective thoughts, emotions, and preconceptions to achieve a perfect, instinctive unity with the car. Sakamoto is a staunch disciple of this approach and holds Ikeda in the highest regard, viewing him not just as a leader but as the sole individual to have truly mastered the principle of becoming one with the machine. His personality is respectful and observant, often providing thoughtful commentary on unfolding events rather than being a central combatant in the races themselves.

Within the story's structure, Sakamoto's primary role is as a supporting member of the opposing team during Project D's远征 (expedition) to the Kanagawa prefecture. He is first seen alongside Ikeda and other teammates at the Nanamagari course when Project D arrives for their scheduled battle. He offers his impressions of the visiting team, acknowledging their formidable presence and noting their lack of arrogance despite their long winning streak. This interaction serves to establish the serious and competitive, yet sportsmanlike, atmosphere of the encounter. Later, during the uphill race between Keisuke Takahashi and Ikeda, Sakamoto reflects on the team's thoughtful approach to car modification and discusses the principles of the Zero Theory, particularly how it has improved the driving of younger team members. After Ikeda loses the race, Sakamoto initially protests the outcome, arguing for a rematch under clear weather conditions, though his leader gracefully declines, stating that weather is a natural part of public road racing.

Sakamoto's key relationship is undoubtedly with his team leader, Ryuji Ikeda. He acts as a confidant and a sounding board, engaging in philosophical discussions about racing before the battles commence. He seeks to understand Ikeda's meditative state and is a fervent believer in the efficacy of the Zero Theory as taught by him. This loyalty also places him in a supporting role, as seen when he is stationed along the Hakone Turnpike to provide crucial intelligence. He contacts Ikeda by phone to report the location of an oncoming vehicle driven by the Shinigami, or死神 (God of Death), allowing Ikeda to position himself to intervene in a dangerous, high-stakes chase involving Ryosuke Takahashi. This act underscores his role as a reliable and resourceful observer who contributes to the safety and order of the racing community.

In terms of development, Sakamoto's character arc is subtle. He begins as a firm believer in a seemingly invincible theory but is present to witness its imperfections when it is tested by a driver of Keisuke's caliber. Ikeda's admission that the Zero Theory requires refinement marks a moment of growth for the team as a whole, and by extension, Sakamoto processes this setback. His development is not about a dramatic change in personality but rather a deepening of his understanding of racing's complexities. He learns alongside his leader that no theory is absolute and that real-world conditions and exceptional opponents can challenge even the most well-conceived philosophies. He remains a devoted member of Spiral ZERO throughout the Kanagawa expeditions.

Sakamoto is notable not for spectacular driving feats, as his own race is not depicted in detail, but for his analytical mind and his deep understanding of a specific racing philosophy. The car he drives is not specified in the source material. His abilities lie in his comprehension of the discipline and mental focus required by the Zero Theory. He serves as the narrative's primary voice for explaining and defending this approach, making him an important figure for illustrating the intellectual side of street racing that contrasts with the more instinctual or emotionally driven styles of other characters. His presence highlights that mastery in the world of Initial D is as much a mental and philosophical pursuit as it is a test of physical driving skill.