Koichi Yamadera is a graduate of Tohoku Gakuin University and is affiliated with Across Entertainment. He is a prolific voice actor who specializes in video games and TV animation.

His voice-acting debut was in the OVA Megazone 23 as the motorcycle-riding Shinji Nakagawa, while his TV debut was the anime Bosco Adventure as the cowardly but friendly otter. His break-out voice role was Ranma 1/2, in which he played Ryoga Hibiki and the Jusenkyo Guide. That same year, he voiced Shaman in the TV series Granzort. In 1991, he joined forces with Toshihiko Seki and Noriko Hidaka to form the acting unit Banana Fritters, which disbanded in 1995.

In October 1997, he became a host on the TV Tokyo children's variety show Oha Suta.

He is best known for his roles in Kaiketsu Zorori, the Japanese dub of Full House, Ranma 1/2, Soreike! Anpanman, Brave Exkaiser, Ghost in the Shell, Ghost in the Shell: Innocence, Neon Genesis Evangelion, Cowboy Bebop, the Yatterman remake, Mobile Suit Gundam: Char's Counterattack, Paprika and Vampire Hunter D-Bloodlust.

He is also known for dubbing for Jim Carrey and Eddie Murphy in English-language films.