Yamaguchi's jump float serve improves throughout the series as he practices consistently, refines his technique, and gains confidence in his ability. Initially, he struggles with accuracy and power, but over time he learns to control the speed and placement of his serves. With guidance from his teammates and coaches, Yamaguchi hones his serve to become a valuable asset for the team, often using it strategically to pressure opponents and score points. Overall, his dedication and growth mindset contribute to the progression of his jump float serve throughout the series.