Her father was a bassoon player with [a875118], her mother a classical vocalist. Koshi received an education in classical music and ballet. She gained a recording contract with RCA following an appearance on the TV talent show "君こそスターだ!" ("You Are The Star!") which she won in October 1978. For her early works her name appeared in kanji as 越美晴, but for albums from 1989 to the present day, it has appeared in katakana as コシミハル (both are pronounced the same: Koshi Miharu). Koshi deliberately sought to "de-idolise" her image from the point she met and began collaborating with Haruomi Hosono.