According to the Anime News Network, as of December 2008, Koyasu is the most prolific seiyū in Japan with voice acting roles in 365 anime series. He is also part of the seiyū 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 bishōnen. Recently, though (as in the cases of Mitsurugi Hanagata from Saber Marionette J, Ryo Kuroyanagi from Yakitate!! Japan, Bobobo-Bo-Bobobo, and Kei Itoshiki from Zoku Sayonara Zetsubo Sensei) he has been voicing more and more lively/humorous characters. According to Hisaya Nakajo, the mangaka of the series Hana-Kimi, the character of Masao Himejima was designed with Koyasu's voice in mind; the seiyū ended up voicing this character in the