Koishi Yamamoto is a 62-year-old Japanese referee for the Kengan Association, standing 1.70 meters tall and weighing 100 kilograms with a bald head and black eyes. His primary occupation is officiating professional wrestling matches, where he has accumulated extensive experience over many years, including at least 21 years with the Kengan Association. Known by the nickname "Devil Sergeant," Yamamoto exhibits a tendency to allow matches to continue longer before declaring an end, prioritizing the natural flow of the fight, an approach that sometimes results in severe consequences for the fighters. Despite previously considering retirement, he was convinced by Katahara Metsudo to continue his refereeing career into his seventies.

Yamamoto officiated the following matches: Tokita Ōma vs. Nakata Ichirō; Sekibayashi Jun vs. Tokita Ōma; Adam Dudley vs. Imai Cosmo; Kōno Haruo vs. Akoya Seishū; Kure Raian vs. Mokichi Robinson; Inaba Ryō vs. Tokita Ōma; Imai Cosmo vs. Akoya Seishū; Kure Raian vs. Tokita Ōma; Imai Cosmo vs. Tokita Ōma; Tokita Ōma vs. Wakatsuki Takeshi; and Nakata Ichirō vs. Mokichi Robinson.

Additionally, the anime adaptation specifically depicted Yamamoto refereeing these matches, which had different referees in the manga: Sekibayashi Jun vs. Kiōzan Takeru (manga: Tashiro Mosashi); Yoroizuka Saw Paing vs. Karo Yoshinari (manga: Anna Paula); Nezu Masami vs. Mikazuchi Rei (manga: Anna Paula); Nikaidō Ren vs. Kiryu Setsuna (manga: Anna Paula); Nakata Ichirō vs. Kuroki Gensai (manga: Anna Paula); Chiba Takayuki vs. Hatsumi Sen (manga: Cheetah Hattori); Hanafusa Hajime vs. Bandō Yōhei (manga: Cheetah Hattori); Gaolang Wongsawat vs. Kaneda Suekichi (manga: Cheetah Hattori); and Ōkubo Naoya vs. Kanō Agito (manga: Cheetah Hattori).