Eiji Kimura is a guitarist known for his role in the visual-kei band Belle Ame, as well as for his past as a member of the group Serial Mama. His history is deeply intertwined with Ryusuke Minami, as the two were originally bandmates and guitarists in Serial Mama. The dissolution of that band, triggered by a failed opportunity to perform for a producer due to a technical problem, marked a pivotal turning point. Following this event, Eiji and Ryusuke became rivals, each making a vow to form the greatest band and thereby embarking on separate musical paths.

As a character, Eiji is driven by pride and a fierce competitive nature. His primary motivation is to fulfill his promise to create a band superior to Ryusuke's, and he maintains a persistent grudge, especially when he feels Ryusuke or his band, BECK, is gaining an advantage. He is ambitious and initially finds rapid commercial success with his new band, Belle Ame, which gains popularity through a combination of sound and the members' visual appeal. However, his journey is marked by a series of letdowns, both on national and international stages, which eventually lead him to part ways with Belle Ame to figure out his own path.

Eiji's role in the story is primarily that of a rival to BECK, representing a more commercially viable and mainstream alternative to the protagonists' grittier, more authentic sound. His relationships are largely defined by his rivalry. His dynamic with Ryusuke is the central conflict, characterized by a sour grudge and a competitive race to recruit top talent. This is exemplified by their mutual interest in bassist Yoshiyuki Taira, whom Eiji attempts to recruit first. When Taira ultimately chooses to join Ryusuke's band, the relationship between Eiji and Taira sours from Eiji's perspective, and he later scoffs at Taira for what he perceives as a missed opportunity for success. Eiji also has a close professional relationship with Ran, the head of a record label, to whom he feels he owes his initial success, though he eventually decides to leave the label behind.

Throughout the story, Eiji exhibits notable abilities as a skilled guitarist, but his character development is tied more to his evolving understanding of success and artistic integrity than to his technical skills. Key events in his development include a notable concert where he appears as a special guest, and a major rivalry at the Greatful Sound festival. At this festival, his band Belle Ame plays on the main stage while BECK is relegated to a much smaller one. The resulting competition draws the audience and ends with both groups achieving a similar level of attendance, symbolically bringing their long-standing rivalry to a point of equilibrium. This arc showcases his journey from a commercially driven musician to one who must confront his own limitations and redefine his ambitions outside of his predetermined rivalry.