He made his debut with the team on 5 May 2022 against the Hokkaido Nippon-Ham Fighters at the Belluna Dome. On 7 May, he recorded his first hit against the Hiroshima Toyo Carp.[3] He scored his first RBI on 11 May against the Carp.[4] He hit his first home run on 20 July against the Chiba Lotte Marines at the Zozo Marine Stadium.[5] In 26 games during the season, Koga recorded a .155 batting average, nine hits, one home run and four RBI.
In 2023, he finished the season with a .218 batting average, 52 hits, two home runs and 20 RBI in 100 games. In 2024, he played 105 games, recording a .228 batting average, 56 hits, three home runs and 12 RBI.[6]