Ken Yamaguchi was a Japanese actor and voice actor born Kiyoshige Yamaguchi on March 24, 1956, in Fukushima, Japan. He was active in the industry from the late 1970s until his passing on October 24, 2011, due to acute heart failure at the age of 55. At the time of his death, he was represented by the agency OYS Produce.

Yamaguchi possessed a versatile voice that brought to life a wide array of characters across anime, original video animations (OVAs), and tokusatsu television series throughout the 1980s, 1990s, and 2000s. He was particularly prolific in the OVA market, a format that flourished during the 1980s and 1990s. He was known for voicing many antagonistic or supporting roles in action-oriented and comedy series.

A significant portion of his career was dedicated to long-running and popular franchises. He voiced multiple characters in the Kinnikuman universe, including Ashuraman, Prisman, and The Omegaman. In the Fist of the North Star 2 series, he played the role of Ein. He was also a recurring voice in the various Transformers anime series produced in Japan, providing voices for characters such as Hydra in Transformers: Super-God Masterforce and Blurr in Transformers: The Headmasters and Transformers 2010. His work extended to the Dragon Ball franchise, where he voiced the character Ebifurya in the film Dragon Ball Z: The World's Strongest Guy and Suu Shinron (the Four-Star Dragon) in Dragon Ball GT. In Saint Seiya, he was the voice of Eligor in the film Saint Seiya: Warriors of the Final Holy Battle and the Black Dragon in the television series.

Beyond these major franchises, Yamaguchi's filmography includes a vast number of roles in series and OVAs from the 1980s onward, many of which are noted in the query. His credits include Genji Togashi in Sakigake!! Otoko Juku, Gōta Yamada in Ai no Senshi Rainbowman, Akira Ishikawa in Shōnan Bakusōzoku, Baros Karateya in Armored Fleet Dairugger XV, and Gakurander in Machine Robo: Battle Hackers. He was also a regular presence in the Urotsukidōji series, voicing the character Münchhausen II across multiple installments.

Yamaguchi frequently collaborated with a core group of fellow voice actors, indicating a regular presence in the industry during its peak decades. His frequent co-stars included Daisuke Gōri, Kōji Totani, Takeshi Kusao, Toshio Furukawa, and Masaharu Satō, with whom he worked on numerous projects. His son, Kiyohiro Yamaguchi, is also a voice actor.

Ken Yamaguchi's career is marked by a high volume of work, contributing his voice to a significant number of beloved anime series and OVAs from a formative era of the industry, leaving behind a substantial legacy of character roles.