Kodai Kumasaka was born on 15 April 2001 in Saitama. He developed into a 185cm central midfielder and joined Kashiwa Reysol's senior side. The 2024 season produced 10 J1 League matches; 2025 brought 18 J1 matches plus a J.League Cup appearance.
For Japan, Kumasaka has been a frequent friendly squad member, with 18 senior friendly appearances in 2025.
A 185cm two-footed central midfielder with vision, height for set pieces and the technical security to play out from defence, Kumasaka is one of Kashiwa's senior central midfielders. He is in the wider 2026 FIFA World Cup squad picture as a J-League-based central midfielder option.