She is the 1993 World Junior champion,[1] 1992 NHK Trophy silver medalist, 1997 Winter Universiade champion, and a three-time Japan national bronze medalist.

Koiwai finished 8th at the 1992 World Junior Championships in Hull, Quebec, Canada.

At the 1993 Junior Worlds in Seoul, she placed first in both segments and was awarded the gold medal ahead of Lisa Ervin and Tanja Szewczenko.

Coached by Machiko Yamada, who had trained Midori Ito, Koiwai was routinely landing the triple Axel jump by 1994.

Koiwai graduated from Tokai Women's College near her hometown, Nagoya.