Koyasu is known for his deep, smooth voice, which often lands him on roles that are either villains or lonely bishōnen. Occasionally, 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 voices lively 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 series' drama CDs. Intrestingly, both he and Charles Rolfe did the voice of Rezo the Red Priest in Slayers and the voice of A'sharda Kagewaki (after Daiki Nakamura) in Ranma 1/2.
Koyasu is also the creator of the Weiß series, and has authored the manga Weiß Side B. In creating Weiß Kreuz, Koyasu was inspired by an old Japanese series called Sure Death.
Samurai Gakuen (Hotaru) Onmyouden no Tobira (Hotaru) Pink na Chopin