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.
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.
