Fuyuki Yamamoto, 34, manages the idol group Mystery Kiss for his talent agency. He prioritizes the group's success above all, imposing strict rules including dietary restrictions on member Yuki Mitsuya, threatening termination if she gained five kilograms. Yamamoto considers Rui Nikaidou the group's "pure gem," viewing Mitsuya and Shiho Ichimura primarily as supports to enhance Nikaidou's appeal, believing groups are more marketable than solo artists.

On October 4, Yamamoto discovered Mitsuya dead at a meeting site intended for her and Nikaidou. To conceal her death and protect Mystery Kiss' future, he enlisted yakuza members Haruhito Yano and Togo Sekiguchi to dispose of the body and personally cleaned the scene. In exchange, Yamamoto promised Yano half of Mystery Kiss' future profits and obligated the group to perform tasks for the yakuza.

To maintain the group's public image, Yamamoto recruited Sakura Wadagaki to impersonate Mitsuya. He enforced a mask policy requiring Wadagaki and Ichimura to wear masks during all performances to hide the substitution. Intensive training brought Wadagaki's performance near Mitsuya's original standard, though Nikaidou expressed dissatisfaction with the group's direction during this period.

Confronted by Hiroshi Odokawa about his role in the cover-up and related crimes, Yamamoto reacted violently, attempting to strangle Odokawa after Odokawa revealed his knowledge and tried to leverage the information. Yamamoto's actions progressed from dedicated management to compromised ethics and criminal behavior to preserve Mystery Kiss' commercial viability.