After graduating from high school in 2021, he was didn't receive any contract offer from professional clubs, leading him to enter the Takushoku University.

[2] In May 2023, Kashiwa Reysol announced the signature of Sekine from 2025 and registered him in the squad as a "specially designated player".

[5] He quickly gain a starter spot at the club, starting in all 7 J1 League games before leaving the team to participate in the U-23 Asian Cup.

Leo Kokubo later saved the penalty shot from Umarali Rakhmonaliev, securing the title for Japan.

[10] In October 2024, he received his first call up to the Japan national team, replacing the injured Kota Takai for the 2026 FIFA World Cup qualification matches against Saudi Arabia and Australia.