A left-handed batter and right-handed thrower, Kotaro stands 6 feet 0 inches (1.83 m) tall and weighs 225 pounds (102 kg).

Kiyomiya drew major news attention in the 2012 Little League World Series, where at the age of 13 he employed an 80 mph fastball (major league equivalent of throwing 104 MPH) to lead Team Japan to a 12–2 LLWS championship win over the team from Goodlettsville, Tennessee.

On August 23, 2017, it was announced that he would serve as captain of Samurai Japan for the 2017 U-18 Baseball World Cup.

Seven teams chose Kotaro in the first round of the 2017 NPB draft, the Hokkaido Nippon-Ham Fighters won the right to negotiate with Kiyomiya by winning the lottery.

[3] On February 27, 2019, he was selected for Japan national baseball team at the 2019 exhibition games against Mexico,[4] but on March 4, 2019, he canceled his participation due to broken right hamate bone.