Mitsuo Yamaguchi (山口 光雄, Yamaguchi Mitsuo, born May 23, 1965), better known by the stage name of Kappei Yamaguchi (山口 勝平, Yamaguchi Kappei), is a Japanese actor and voice actor affiliated with Gokū and 21st Century Fox. He currently voices Usopp in One Piece and Shinichi Kudo in Case Closed. Other roles included the male Ranma Saotome in Ranma ½, the eponymous character in Inuyasha and L in Death Note. In overseas dubs, he is the Japanese voice of Kyle Broflovski, Bugs Bunny and Crash Bandicoot.
Yamaguchi appeared in eroge as Kyōya Ushihisa (牛久 京也, Ushihisa Kyōya). He made his first public appearance in North America at Otakon 2008 and in Sakura-Con 2009. His third appearance to date has been at Animazement in 2010. He also made a fourth appearance in Hawaii at Kawaii Kon in 2011. He is married and has a son and a daughter.
He is also known as Mitsuo Yamaguchi and Kyōya Ushihisa.