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 series' drama CDs.

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.

Koyasu often voices under the pseudonym Hayato Jumonji, especially in H anime.

He also narrates promotional videos and pre-flight safety videos for StarFlyer.

Blood type: A

Height: 175 cm

Weight: 60 kg

Bust-Waist-Hip: 83-65-93 cm

Favourites:

Beverages: milk, brandy

Colours: black, white, blue

Country: Egypt

Animal: Giraffe

Brands of clothing: Fila, Donna Karan, Van, Ships

Date spot: the sea

Music group: L'arc~en~Ciel

TV show: "trendy" dramas

Flower: yellow rose

Foods: ebi, tofu, men's pocky

Idol: Mita Hiroko

Season: autumn

Hobbies:

Snowboarding

Video Games

Writing

Skills & Abilities:

Baseball

Designing signatures

E Rank Driver

Jump rope

Photography