Kappei Yamaguchi, born Mitsuo Yamaguchi on May 23, 1965, in Fukuoka, Japan, is a highly esteemed Japanese actor and voice actor. He has been active in the entertainment industry since 1985 and is affiliated with his own production company, Gokū, as well as the 21st Century Fox theater company. His path to voice acting began after moving to Tokyo at the age of nineteen to attend the Tokyo Announce University College, a vocational school for performers. He further honed his skills in acting classes taught by Kaneta Kimotsuki and gained valuable stage experience as a member of the 21st Century Fox theater company.

Yamaguchi's career is marked by an extraordinary number of iconic roles, often lending his distinctive, high-pitched voice to lively, passionate, and youthful characters. His debut role in 1989 as the male Ranma Saotome in the beloved series Ranma ½ immediately established him as a prominent voice actor. This was followed by other defining performances in the 1990s and 2000s, including the titular half-demon in Inuyasha and its sequel Yashahime: Princess Half-Demon, the mysterious and genius detective L in Death Note, and the sharpshooter Usopp in the long-running series One Piece. He is also a permanent fixture in the Case Closed franchise, voicing both the original teen detective Shinichi Kudo and the charismatic phantom thief Kaito Kid (Kaito Kuroba). Beyond these, his filmography includes a vast range of characters such as Teddie in Persona 4, Sai Saici in Mobile Fighter G Gundam, and Jin in YuYu Hakusho.

In addition to his work in Japanese animation, Yamaguchi is the regular Japanese dubbing voice for several iconic Western characters, including Bugs Bunny from the Looney Tunes franchise, Kyle Broflovski from South Park, and the video game character Crash Bandicoot. His versatility allows him to also take on more serious or low-key roles by lowering his vocal register, as seen in projects like Record of Lodoss War. Throughout his decades-long career, he has occasionally used the pseudonym Kyōya Ushihisa for work in adult-oriented projects.

Yamaguchi's talent and contributions to the industry have been recognized with several awards. He won the individual seiyu award at the 2004 Tokyo International Anime Fair. In 2019, he was honored with the Kei Tomiyama Award at the 13th Seiyu Awards, which recognizes a voice actor who has made a significant impact in the industry through their body of work. As of March 2006, he was tied as the third most prolific voice actor in Japan. Beyond voice acting, Yamaguchi is also a skilled rakugoka (comic storyteller), having begun his formal study of the traditional Japanese performance art in 2017 and performing under the name Nonoya Pepepee. He is also an illustrator and has shared his artwork in publications related to anime and voice acting. He is married and has two children, a son, Ryunosuke, and a daughter, Akane, who have both followed him into the voice acting profession.