is a Korean-Japanese professional rugby union player who plays as a prop for Japan Rugby League One club Kobelco Kobe Steelers. Born in South Korea, he represents Japan at international level after qualifying on residency grounds.
He also changed his nationality to Japan after the 2019 World Cup.
After only two Super Rugby appearances for the Sunwolves, which included one start, Koo received his first call-up to his adopted country, Japan's senior squad ahead of the 2017 end-of-year rugby union internationals.