Reiji Namikawa is the leader of the First Sales Division at the Yotsuba Group, a major corporation, and is a central figure among the eight executives who secretly convene to use the power of Kira for corporate gain. At thirty years old, Namikawa is the youngest member of this group and stands out for his charisma, sharp intellect, and composed demeanor. He is noted for his striking appearance, with long, smooth black hair, and is often described as elegant and attractive, earning him the nickname Sephiroth among his colleagues.

Born on August 3, Namikawa comes from a wealthy and connected background; his father is the president of Yotsuba America. He spent six years living in the United States and earned a degree in business administration from Harvard University, a prestigious educational pedigree that complements his natural strategic mind. His personality is defined by a cool, decisive nature and a deep respect for efficiency, holding a strong disdain for incompetent or inefficient subordinates. While he is businesslike and often appears detached, he possesses exceptional communication and leadership skills, which make his opinions highly influential during the group's secret meetings.

Namikawa's primary motivation is professional success, but he is pragmatic rather than blindly ambitious. Unlike some of his peers, he does not believe the company needs to rely on Kira to achieve its goals, confident that he could succeed through his own talent alone. This independent mindset shapes his role in the story. When Light Yagami, disguised as L, contacts him and reveals that the investigation team can monitor the Yotsuba meetings, Namikawa is the one who accepts the proposal to halt the killings for one month. He successfully uses his persuasive skills to convince the other executives to agree, effectively putting the group's murderous activities on hold to protect them from exposure.

His relationships with the other Yotsuba members are defined by his status as a natural leader. He is respected by colleagues such as Suguru Shimura, who believes Namikawa is destined for a top position in the company. Along with Shingo Mido and Shimura, Namikawa privately deduces that Kyosuke Higuchi is the true Kira within their group, using logical elimination to identify their dangerous colleague. The detective L also explicitly recognizes Namikawa's high intelligence, singling him out as a person of note. In the climax of the Yotsuba arc, Namikawa plays a key role by tricking Higuchi into watching a specific television broadcast, which directly facilitates Higuchi’s capture.

In terms of abilities, Namikawa is an expert chess player, possessing skills comparable to a professional fourth-dan player. This hobby reflects his strategic and analytical nature, which he applies to both business and the dangerous situation surrounding Kira. His development throughout the arc shows a man navigating a lethal game; he begins as a participant in the killings but quickly shifts to self-preservation when the group is threatened, ultimately cooperating with L to stop Higuchi. Despite his survival through the Yotsuba incident, Namikawa is eventually killed by Light Yagami, collapsing after a later meeting alongside the remaining former members of the group, as Light eliminates all loose ends.