In an interview Yamaguchi explains that he first decided to become an actor in high school when he suddenly announced the idea during a parent-teacher conference but, in retrospect, he knew nothing about acting at the time and possibly just wanted a reason to move to Tokyo. ( Interview Source )
He joined a voice acting school where his teacher would often recommend live theater, and ultimately joined his teacher’s theater troupe. (His teacher was Kimotsuki Kaneta, who had roles in projects such as Astro Boy, Doraemon, and Galaxy Express 999, and dubbed characters such as Scuttle in The Little Mermaid, Jiminy Cricket in Pinocchio and Friar Tuck in Robin Hood.)( Interview Source )
Yamaguchi said that he supported himself through part-time jobs while a member of the theater troupe, receiving help from his sister but also working as a mascot-character (where he first met Hidaka Noriko, with whom he would eventually collaborate on Ranma 1/2), delivering newspapers, and working at restaurants, often deciding where to work based on what uniforms he liked. ( Interview Source 1 | Interview Source 2 )
He began voice acting at the of 23 after he was scouted by anime sound director Shiba Shigeharu, who reached out to him after seeing a live stage performance in which Yamaguchi had just one line of dialogue. ( Interview Source 1 | Interview Source 2 )
Yamaguchi’s debut role was lead character Saotome Ranma in Ranma 1/2. His most famous roles include Inuyasha (Inuyasha), L (Death Note), Usopp (ONE PIECE), Sakuma Ryuichi (Gravitation), Subaru (Tokyo Babylon), and Shinichi Kudo/Kaito Kuroba (Detective Conan).
His other projects include:
・100 Sleeping Princes & The Kingdom of Dreams (with VAs for Baul, Ortho, Leona, Floyd, Lilia, Malleus, Trein, Riddle, Grim, Fellow, Neige, Ace, Azul, Rollo)
・Angel’s Feather
・Apocripha/0
・Baby & Me
・Betterman
・Brave Command Dagwon
・Bucchigiri?!
・Case Closed
・Danganronpa: The Animation (with VA for Swing)
・Death Note (with VA for Swing)
・Detective Conan: The Million-dollar Pentagram (with VA for Trein)
・Dive in Wonderland (with Riddle’s VA)
・Duel Masters
・Durarara!!x2
・Eden’s Bowy
・Eyeshield 21
・Final Fantasy VII Rebirth
・Final Fantasy VII Remake (with Fellow’s VA)
・Flint the Time Detective
・Futari wa Pretty Cure Splash Star
・Galaxy Express 999
・GeGeGe no Kitarou
・Gods’ Games We Play (with VAs for Crewel, Silver, Ace)
・Granblue Fantasy (with VAs for Idia, Lilia, Baul, Neige, Vargas, Jade, Ace, Floyd, Silver, Azul, Rollo, Deuce, Trein, Sebek, Crewel, Fellow, Grim, Leona, Riddle, Maleanor, Trey, Ortho, Yuuken)
・Grandia
・Gravitation
・Gulliver Boy
・Gun Frontier
・In Search of the Lost Future
・Inuyasha
・Ishura (with VAs for Lilia, Fellow, Baul, Trein, Floyd)
・Jungle King Tar-chan♡
・Kamiwaza Wanda
・Kiki’s Delivery Service
・Magic Kaito
・Magica Wars
・Manmaru The Ninja Penguin
・Mechanical Arms (with VAs for Fellow, Azul)
・Miracle ☆ Girls
・Monkey Typhoon
・Neighborhood Story
・Neo Angelique・New Pokémon Snap
・Ninja Girl & Samurai Master
・Odd Taxi
・ONE PIECE (with VAs for Floyd, the Director, Baul, Rollo, Trein, Fellow, Grim, Vargas, Swing)
・Persona 4 (with Fellow’s VA)
・Ranma 1/2
・Red Baron
・Rin-ne
・Rizelmine
・Samurai Pizza Cats
・SD GUNDAM BATTLE ALLIANCE
・Shippuu! Iron Leaguer
・The Gymnastics Samurai
・The Heroic Legend of Arslan
・The Law of Ueki
・The Twelve Kingdoms
・Tiger Mask W
・Train to the End of the World (with VA for the director)
・Weiß kreuz Glühen
Twisted Wonderland Reference Note 