According to the Anime News Network, as of December 2008, Koyasu is the most prolific seiy? in Japan with voice acting roles in 365 anime series. He is also part of the seiy? 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 seiyuu 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 bish?nen.