Kōtarō Watari is a second-year student at Sōshūkan High School, widely recognized as Ao High, and an active member of its boys' rhythmic gymnastics team. He joined immediately upon entering high school, inspired by his seniors' backflip performances, becoming one of the team's youngest recruits at the time. As the only second-year gymnast currently on the roster, he specializes in handstand holds during routines.

Watari sports mid-length green hair with a darker green undercut, black eyes, and characteristically slanted eyebrows that lend him an intense expression. Outside practice, he typically wears a blue-and-white tracksuit layered over a green hoodie. His training attire consists of a white long-sleeve shirt beneath a yellow outer shirt, paired with black shorts and dark blue leggings. A headband and multiple piercings serve as his signature accessories.

Despite his sharp demeanor and intimidating look, Watari radiates a caring and enthusiastic personality. He demonstrates fierce devotion to his teammates, particularly his seniors, and readily admits faults with profuse apologies. His passion frequently erupts in loud, energetic exclamations during training or conversations.

His family runs a ramen shop, where he honed considerable cooking skills. Within the team, Watari actively nurtures dynamics by supporting underclassmen and strengthening bonds across all members.