[1] Koiwai was born in Kitahiroshima, Hokkaido in 1998 and started playing golf when she was 8 years old.

[2] Koiwai turned professional and joined the LPGA of Japan Tour in 2017.

In 2018, she recorded four runner-up finishes to rank 8th on the money list, and won the PGA Rookie of the Year award and the Japan Professional Sports Rookie of the Year award.

[3] She captured the 2019 Samantha Thavasa Girls Collection Ladies Tournament for her maiden win on the JLPGA.

[1] In mid-2021, she rose into the top-50 in the Women's World Golf Rankings for the first time.