Koichi Yamadera, born June 17, 1961, in Shiogama, Miyagi Prefecture, is a highly esteemed Japanese actor, voice actor, and narrator. He is a graduate of Tohoku Gakuin University's economics school and is currently affiliated with Across Entertainment. Yamadera's career began in 1985, with his voice acting debut in the original video animation Megazone 23 as Shinji Nakagawa, followed by his television anime debut in Bosco Adventure as the character Otter. His versatility and extensive range have earned him nicknames such as "the man with the seven-colored voice".

Yamadera's breakthrough role came with the anime Ranma ½, where he voiced the dual roles of Ryōga Hibiki and the Jusenkyō Guide. He gained international acclaim for his portrayal of the charismatic bounty hunter Spike Spiegel in Cowboy Bebop. Other seminal anime roles include Ryoji Kaji in Neon Genesis Evangelion, Togusa in the Ghost in the Shell franchise, and Beerus, the God of Destruction, in the Dragon Ball series, starting with Dragon Ball Z: Battle of Gods. He also voices the character Cheese in the long-running children's series Soreike! Anpanman. In 2011, Yamadera succeeded Gorō Naya as the official voice of Inspector Koichi Zenigata in the Lupin III series.

His filmography is vast, encompassing numerous other significant parts, including Shoyo Yoshida / Utsuro in Gintama, Zorori in Kaiketsu Zorori, Donquixote Rosinante in One Piece, Shun Akiyama in the Yakuza video game series, and Sukeroku in Shōwa Genroku Rakugo Shinjū. Yamadera is also known for voicing multiple characters within a single work, a skill prominently displayed in the video game Danganronpa V3: Killing Harmony, where he voiced all five Monokuma Kubs, and in the Japan Animator Expo series of short films, where he and Megumi Hayashibara voiced all characters.

Beyond anime, Yamadera is a prolific figure in Western media dubbing. He is the official Japanese voice for Disney characters such as Donald Duck, Genie, and Stitch. He is also the primary dubbing actor for Hollywood stars including Will Smith, Eddie Murphy, and Jim Carrey. From October 1997 to March 2016, he was a regular host on the TV Tokyo children's variety show Oha Suta. His contributions to the voice acting industry have been recognized with awards, including the Kei Tomiyama Award at the Third Seiyu Awards in 2009 and the Seiyu Award for Best Foreign Film/Drama Dubbing in 2020. As of 2024, he continues to be active, reprising his role as Ryōga Hibiki in the new Ranma ½ anime adaptation.