Kōtarō Watari attends Ao High School in Miyagi Prefecture as a second-year student and competes on the boys' rhythmic gymnastics team. His passion for the sport ignited after watching senior Masamune Shichigahama execute a backflip during a club tour, compelling him to join.

He sports mid-length green hair with a darker green undercut, small green pupils, and often furrowed eyebrows that create an imposing first impression. Piercings and a regular headband accentuate this look. Off the mats, he typically layers a green hoodie under a blue-and-white tracksuit. During gymnastics, he pairs black shorts and dark blue leggings with a yellow shirt layered over a white long-sleeve.

Though his brash exterior unnerves newcomers, Kōtarō harbors a genuinely caring and enthusiastic nature. He demonstrates deep respect for seniors and readily apologizes for errors. His protective instincts surface through supportive interactions with junior teammates, and he expresses excitement with loud outbursts.

Athletically, he specializes in balance and static techniques, particularly handstand holds, strengthening the team's performances. Beyond gymnastics, his family's ramen shop background cultivated notable cooking skills. Personal interests include yakuza-themed films, and his dorm displays chivalry-inspired decorations.