In the 2017 season, Kai mainly teamed up with young starter pitchers such as Kodai Senga, Nao Higashihama, Shuta Ishikawa, and Yuki Matsumoto.

[9] Kai finished the regular season having made 103 appearances and batting .232 with five home runs and 18 RBI.

[15] In the 2019 season, Kai made 137 appearances for Fukuoka, batting .260 with 11 home runs and 43 RBI, as a regular catcher of the Hawks.

On October 31, he was selected as a 2019 Pacific League Mitsui Golden Glove Award winner.

[19] He supported the pitchers as the startng catcher and finished the regular season with a batting average of .211, 11 home runs, 33 RBI, 44 stolen bases, and 22 sacrifice bunts in 104 games.

Kai recorded more than 10 home runs for the second consecutive year, becoming the first catcher to achieve the feat since Kenji Jojima, and his 22 sacrifice hits were the most in the Pacific League in 2020.

[22][23][24] In 2021, Kai played in all 143 games and finished the year with a .227 batting average, 12 home runs and 44 RBI.

[25] On December 16, 2021, Kai was honored with a Pacific League Golden Glove Award for the fifth consecutive season.

[25] The next day, he re–signed with the Hawks for an estimated annual salary of 210 million yen, making him the highest paid active catcher.

[26] On June 28, 2022, Kai tested positive for COVID-19 and was removed from the roster by regulation, but returned on July 17.

[32] Kai played 139 games, and finished the season with a .202 batting average, 10 home runs, and 44 RBI.