Koyasu is part of the voice actor quartet Weiß consisting of Weiß Kreuz voice castmates Shinichiro Miki, Tomokazu Seki, and Yuuki Hiro. He used to work for Production Baobab but is now the head of T's Factory, a voice actor company he founded in 1998.Koyasu is known for his deep, smooth voice, which often lands him in roles that are either villains or lonely bishōnen. Recently, though (as in the cases of Mitsurugi Hanagata from Saber Marionette J, Ryo Kuroyanagi from Yakitate!! Japan, Bobobo-bo Bo-bobo, and Kei Itoshiki from (Zoku) Sayonara, Zetsubou-Sensei) he has been voicing more and more lively/humorous characters. According to Hisaya Nakajo, the manga artist of the series Hana-Kimi, the character of Masao Himejima was designed with Koyasu's voice in mind ; the voice actor ended up voicing this character in the series ' drama CDs.On 18 September 2009, he created Velvet Underworld series which is the first drama CD of the series, "The Fool". In the following month, "The Wheel of Fortune" had released as the second drama CD of the series.
1987The Three Musketeers Anime, Francois1989Ranma ½, Daisuke1990Moomin, Snufkin, The Joxter1991Adventures of the Little Mermaid, Prince Justin
Delightful Moomin Family : Adventure Diary, Snufkin1992The Brave Fighter of Legend Da-Garn, Seven Changer
Tekkaman Blade, Shinya Aiba/Tekkaman Evil
YuYu Hakusho, Asato Kido1993The Brave Express Might Gaine, Ran
Mobile Suit Victory Gundam, Liole Sabat1994DNA², Ryuji Sugashita
Brave Police J-Decker, Victim O'Rand, Mecha Victim
Mobile Fighter G Gundam, Neo Sweden Support Team
Macross 7, Gamlin Kizaki1995Wedding Peach, Sandora
Fushigi Yûgi, Hotohori
Slayers, Rezo the Red Priest, Copy Rezo
Mobile Suit Gundam Wing, Milliardo Peacecraft / Zechs Merquise
Neon Genesis Evangelion, First Lieutenant Shigeru Aoba1996Rurouni Kenshin, Jinpu
Crayon Shin-chan, Honda, Kakeru Futamata
Brave Command Dagwon, Kai Hirose
Saber Marionette J, Mitsurugi Hanagata1997Pocket Monsters, Kosaburō
Revolutionary Girl Utena, Kiryuu Touga1998Trigun, Roderick Captain
Initial D, Ryosuke Takahashi
Nazca, Masanari Tate
Sorcerous Stabber Orphen: Begins, Flameheart
Weiß Kreuz, Ran "Aya" Fujimiya1999Betterman, Lamia
Shin Hakkenden, Kai
Turn A Gundam, Gym Ghingham
Angel Links, Warren
Magic User's Club, Ayanojyo Aburatsubo
Pocket Monsters : Episode Orange Archipelago, Kosaburō
Pocket Monsters : Episode Gold & Silver ), Kosaburō
Sorcerous Stabber Orphen: Revenge, Flamesoul
Excel Saga, Il Palazzo2000Gate Keepers, Chotaro Banba
Boys Be..., Takuya Yokota
Sakura Wars, Yuichi Kayama
Yu-Gi-Oh! Duel Monsters, Pandora
Gravitation, Sakano
Argento Soma, Dan Simmonds2001Beyblade, Boris Balkov (Borkov)
Star Ocean EX, Dias Flac
Detective Conan, Shiro Murakami (ep. 248)
Prétear, Tanaka
Inuyasha, Gatenmaru
Shaman King, Faust VIII
Final Fantasy : Unlimited, Pist Shaz the 11th
Captain Tsubasa : Road to 2002, Adult Kojiro Hyuga
Hellsing, Luke Valentine2002Full Metal Panic!, Zaiede
Pocket Monsters Side Stories, Kosaburō
Pocket Monsters : Advanced Generation, Kosaburō
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