By chance, he saw the scene before Tetsurō Hoshino's departure in Adieu Galaxy Express 999 where a group of nameless soldiers take him on the 999 and send him on his way, and was impressed by it.
[7] In 1988, he made his voice acting debut in Wowser, and in 1989, he played his first regular character as Gai, the King Yaksha in Legend of Heavenly Sphere Shurato.
Although he was a regular character in the series, he was in the position of a villain whose inner workings were rarely discussed in the story, so it was difficult for Koyasu to grasp the role and he had to do several dozen retakes for a single line.
The OVA was based on a fantasy manga of the same name that was being serialized at the time, and the voice actors for the main characters were to be decided by a magazine reader's vote.
Since Koyasu had appeared in Shurato shortly before this, he had some recognition among the viewers, and he received the most votes for the role of Hades, the main character.
Koyasu himself has often said in interviews that this is his favorite character, and after a period of sluggish performance, he put his voice acting career on the line to play the role, which ultimately led to his recovery.