Kosei was born in Japan but raised in Christchurch and started playing rugby from the age of 6. At 19, he took the opportunity to play professional rugby in Japan and made his RWC debut in the 2007 RWC. Throughout a 14 year professional career, he played for the Munakata Sanix Blues and Tokyo Suntory Sungoliath. Capped for Japan 34 times, he also played a part in the highly successful 2015 RWC team.
Since retiring from professional sport, Kos has joined Halo Sport as a Talent Manager. His aspiration it to not only assist players throughout their successful rugby careers in Japan but also to highlight to them how to enjoy their time in this country and to fully embrace the Japanese culture.