Competing in the 2016 and 2017 seasons, he finished sixth and fifth in the standings, respectively. In 2017, he also completed his first full season in the Junior World Championship, but it was in 2018 that he secured his first FIM CEV Repsol victory. That same year, he debuted in the Red Bull MotoGP™ Rookies Cup, with a standout win at Mugello.

Aiming for championship success in both series in 2019, Kunii’s ambitions were hindered by injuries, though he still managed to claim victories in both competitions. His performances earned him a Moto3™ World Championship seat with Honda Team Asia in 2020. A challenging rookie season saw him finish without a World Championship point, but he improved in 2021, securing his first point and finishing 25th overall, with his best result coming at the Styrian GP.

Kunii exited the Grand Prix paddock at the end of 2021 but is set to make his return in 2025 after signing with Idemitsu Honda Team Asia to compete in the Moto2™ World Championship.