Kazuki Kaionji is a supporting character in the anime Battery. He is a male middle school student with brown eyes and short brown hair that reaches his ears, often seen wearing the team baseball uniform and his glasses. He serves as the captain of the Nitta East Junior High School baseball team, where he is the fourth batter and the starting shortstop.

On the surface, Kazuki appears to be a serious, quiet, and somewhat plain or unassuming individual. This initial simplicity, however, belies a sharp and strategic mind. He is described as an excellent tactician capable of identifying an opponent's weaknesses without them ever becoming consciously aware of his analysis. As a player, he is an all-rounder, demonstrating competence in both offense and defense, which makes him a highly reliable asset to his team. His abilities as a leader and his solid performance on the field have earned him the deep trust and respect of the other team members.

Kazuki's primary role within the story is as the captain and a stabilizing force for Nitta East's baseball club. He is deeply involved in the team's dynamic, particularly in his interactions with the gifted but challenging pitcher Takumi Harada and the positive catcher Go Nagakura. His personality and position often place him in a key leadership role, handling the complexities of managing individual talents and team cohesion. While his own character arc is more understated compared to the main protagonists, his steady presence and strategic thinking are crucial to the team's functioning and development throughout the series.