Seishiro Nagi (凪なぎ 誠せい士し郎ろう, Nagi Seishirō?) is a contender for the Blue Lock Project and the titular protagonist of the spin-off, Blue Lock - Episode Nagi, who currently plays as a rightwing for England's Manshine City in the Neo Egoist League.

When first arriving at Blue Lock, Nagi was a member of Team V and was the best player in his designated stratum. After the Blue Lock Eleven played against the Japan U-20 team, Nagi ranked as one of the top players in the project.