Kōichi Yamadera is one of the most popular voice actors in the Pocket Monsters anime series in Japan. He has voiced a character in all the Pocket Monsters movies. His nickname is Yama-chan (山ちゃん) and he hosts the morning kids variety show Oha Suta (おはスタ). From 1993 to their divorce in 2006, he was married to fellow Pokémon voice actress Mika Kanai. He would remarry in 2012 with another fellow actress, Rie Tanaka, but would end up getting divorced with her as well. He would remarry once more in 2021 to singer Robin Shōko Okada.

After the announcement airing on Oha Suta on July 31st, 2020, the official movie website updated with comments from him about his role as Celebi and Professor Zed in Movie 23.

Celebi, Professor Zed. Voice: Kouichi Yamadera

Celebi, the Mythical Pokémon known as the guardian god of the forest. Professor Zed, a professor of a Biotope Company who researches the secrets of the forest.

Comment: I was happy to be offered to work in the Pokémon movie again this year. In the past few years, I've alternated between playing a human and a Pokémon, but I'm reassured by the fact that my character, Professor Zed, has Karen, played by Ms. Nakagawa, as his assistant. I saw the trailer for the film, and from the footage I saw, I was excited to see that director Yajima is going to bring a new style to the film again. I want to be one of the first to see the movie!

セレビィ 森の守神と言われる幻のポケモン。 ゼット博士 森の秘密を研究する「ビオトープ・カンパニー」の博士 声｜山寺宏一 Comment 今年もポケモン映画にオファーを頂けて嬉しかったです。ここ数年は、人間の役とポケモンの役を交互に演じさせて頂きましたが、本作で僕が演じるゼッド博士には中川さん演じるカレンが助手としてついているようなので心強い限りです。予告編を拝見させてもらいましたが、映像を見たところ、今回も矢嶋監督が新しい風を届けてくれそうだなとワクワクしました。僕自身もいち早く映画を観たいです！

Role	Media	Title
Voice Actor Mew Uncredited	Video Game	Pokémon Snap
Voice Actor Mew Uncredited	Video Game	Super Smash Bros.
Voice Actor Mew Uncredited	Video Game	Super Smash Bros. Melee
Voice Actor Mew	Video Game	Pokémon Play It! 2
Voice Actor Detective Pikachu	Video Game	Detective Pikachu Returns
Voice Actor Harry Goodman	Video Game	Detective Pikachu Returns
Voice Actor Keith's Farfetch'd	SL 49	A Sitting Duck for Kamonegi   カモネギのカモ   So Near, Yet So Farfetch'd
Voice Actor Mew	Movie 1	Mewtwo Strikes Back   ミュウツーの逆襲   Pokémon the First Movie - Mewtwo Strikes Back
Voice Actor Lugia	Movie 2	The Explosive Birth of the Mythical Pokémon Lugia   幻のポケモン ルギア爆誕   Pokémon the Movie 2000 - The Power of One
Voice Actor David	Movie 3	The Emperor of the Crystal Tower, Entei   結晶塔の帝王 ＥＮＴＥＩ   Pokémon 3: The Movie: Spell of the Unown
Voice Actor Celebi Poacher	Movie 4	Celebi A Timeless Encounter   セレビィ時を超えた遭遇   Pokémon 4Ever: Celebi, the Voice of the Forest
Voice Actor Ross	Movie 5	The Guardian Gods of the Water Capital Latias and Latios   水の都の護神 ラティアスとラティオス   Pokémon Heroes
Voice Actor Butler	Movie 6	Wishing Star of the Seven Nights Jirachi   七夜の願い星 ジラーチ   Pokémon Jirachi: Wish Maker
Voice Actor Professor Lund	Movie 7	The Sky-Rending Visitor Deoxys   裂空の訪問者 デオキシス   Pokémon: Destiny Deoxys
Voice Actor Aaron	Movie 8	Mew and the Hero of the Wave - Lucario   ミュウと波導の勇者 ルカリオ   Pokémon Lucario and the Mystery of Mew
Voice Actor Jack Walker	Movie 9	Pokémon Ranger and the Prince of the Sea - Manaphy   ポケモンレンジャーと蒼海の王子 マナフィ   Pokémon Ranger and the Temple of the Sea
Voice Actor Baron Alberto	Movie 10	Dialga VS Palkia VS Darkrai   ディアルガVSパルキアVSダークライ   Pokémon The Rise of Darkrai
Voice Actor Newton Graceland	Movie 11	Giratina and the Sky's Bouquet: Shaymin   ギラティナと氷空の花束 シェイミ   Giratina and the Sky Warrior
Voice Actor Marcus	Movie 12	Arceus - Towards the Continuum of Conquest   アルセウス超克の時空へ   Pokémon: Arceus and the Jewel of Life
Voice Actor Goone	Movie 13	The Ruler of Illusions Zoroark   幻影の覇者 ゾロアーク   Pokémon Zoroark: Master of Illusions
Voice Actor Mannes	Movie 14	Victini and the Light Hero - Reshiram / Victini and the Dark Hero - Zekrom   ビクティニと白き英雄レシラム / ビクティニと黒き英雄ゼクロム   Pokémon The Movie White Victini and Zekrom / Pokémon The Movie Black Victini and Reshiram
Voice Actor Cobalion	Movie 15	Kyurem VS the Sacred Swordsman Keldeo   キュレムVS聖剣士ケルディオ   Kyurem vs. The Sword of Justice
Voice Actor Genesect Army (Red)	Movie 16	Extremespeed Genesect: The Awakening of Mewtwo   神速のゲノセクト ミュウツー覚醒   Genesect and the Legend Awakened
Voice Actor Ninja Riot	Movie 17	The Cocoon of Destruction and Diancie   破壊の繭とディアンシー   Diancie and the Cocoon of Destruction
Voice Actor Hoopa (Unbound)	Movie 18	The Archdjinni of Rings: Hoopa   光輪の超魔神 フーパ   Hoopa and the Clash of Ages
Voice Actor Alva	Movie 19	Volcanion and the Ingenious Magearna   ボルケニオンと機巧のマギアナ   Volcanion and the Mechanical Marvel
Voice Actor Mayor Oliver	Movie 21	Everyone's Story   みんなの物語   The Power of Us
Voice Actor Zeraora (M21)	Movie 21	Everyone's Story   みんなの物語   The Power of Us
Voice Actor Celebi	Movie 23	Coco   ココ   Pokémon the Movie: Secrets of the Jungle
Voice Actor Doctor Zed	Movie 23	Coco   ココ   Pokémon the Movie: Secrets of the Jungle
Voice Actor Mewtwo (Detective Pikachu)	Movie 99	Pokémon Detective Pikachu   ポケモン名探偵ピカチュウ   Pokémon Detective Pikachu
Voice Actor Detective Pikachu	SP 18	Detective Pikachu ~Splendid Morning Routine~   名探偵ピカチュウ ~華麗なるモーニングルーティン~   Detective Pikachu and the Mystery of the Missing Flan
Voice Actor Doctor Zed	Special 129	The First Ever Terrestrial Broadcast of the Latest Pokémon Movie, "Pocket Monsters the Movie: Coco"!   ポケモン映画最新作「劇場版ポケットモンスター　ココ」地上波初放送！
Voice Actor Celebi	Special 129	The First Ever Terrestrial Broadcast of the Latest Pokémon Movie, "Pocket Monsters the Movie: Coco"!   ポケモン映画最新作「劇場版ポケットモンスター　ココ」地上波初放送！
Voice Actor Remo	XYZ 34	Semifinal Full Battle! Satoshi VS Shota!!   準決勝フルバトル！サトシ対ショータ！！   Analysis Versus Passion!
Voice Actor Zeraora Uncredited	SM 100	The Lightning that Blocks Wind! Its Name is Zeraora!!   風を断つ稲妻！その名はゼラオラ！！   Battling the Beast Within!
Voice Actor Dia	SM 100	The Lightning that Blocks Wind! Its Name is Zeraora!!   風を断つ稲妻！その名はゼラオラ！！   Battling the Beast Within!
Voice Actor Zeraora Uncredited	SM 101	Fire it Off! The Twin Sparking Gigavolt of Friendship!!   放て！友情のツインスパーキングギガボルト！！   Parallel Friendships!
Voice Actor Dia	SM 101	Fire it Off! The Twin Sparking Gigavolt of Friendship!!   放て！友情のツインスパーキングギガボルト！！   Parallel Friendships!
Voice Actor Dia	SM 140	Akuziking Attacks! The Decisive Z-Move Battle!!   アクジキング襲来！Zワザ大決戦！！   Z-Move Showdown!
Voice Actor Mew	PM2019 1	Pikachu is Born!   ピカチュウ誕生！   Enter Pikachu!
Voice Actor Mew	PM2019 20	Go Towards Your Dream! Satoshi and Go!!   夢へ向かってゴー！サトシとゴウ！！   Dreams Are Made of These!
Voice Actor Mew	PM2019 46	Battling & Getting! Mewtwo Comes Back   バトル＆ゲット！ミュウツーの復活   Getting More Than You Battled For!
Voice Actor Mew	PM2019 134	The Future I Seize!   つかみとる未来！   In the Palms of our Hands!
Voice Actor Remo	MES 1	Pocket Monsters XY Special: The Strongest Mega Evolution ~Act I~   ポケットモンスター XY 特別編 最強メガシンカ　～Act I～   Pokémon: Mega Evolution Special I
Voice Actor Narration	PMD 3	Pokémon Mystery Dungeon - Explorers of the Sky ~The Final Time and Darkness Adventure~   ポケモン不思議のダンジョン 空の探検隊～時と闇をめぐる最後の冒険～   Pokémon Mystery Dungeon Explorers of Sky - Beyond Time & Darkness
Voice Actor Mew	PMRAD 4	The Mythical "Mew"   幻の「ミュウ」