Shū Terafune is a high school student with orange or light brown hair styled in a textured fringe, red eyes, and a height placing him between Minami Tenjin and Akari Watanabe. He typically wears his school uniform casually: an untucked blue button-up revealing a black V-neck underneath, dark blue slacks, brown shoes, and a red wristwatch on his right wrist. A left-hander, he uses chopsticks with his left hand and wears his watch on his non-dominant arm.

He acts self-absorbed, conceited, and presumptuous, often irritating others through poorly timed or inappropriate actions. His intense crush on Akari Watanabe magnifies these traits, leading him to neglect duties at his part-time café job to give her excessive drink toppings or attempt inviting her to karaoke despite others urging her to return home to reconcile with Jirō Yakuin. He frequently misreads situations involving Akari, like trying to console her after Minami's absence during a festival, only to be stopped by Sachi Takamiya. He harbors jealousy toward Minami's effortless popularity with girls and resents Jirō for his close relationship with Akari, declaring "war" on Jirō for her affection. He also exhibits perverted tendencies, selecting work shifts to serve attractive girls and possessing pornography on his tablet, though he denies such behavior publicly.

Despite his flaws, he shows genuine concern for Akari's well-being and demonstrates maturity during the sports festival by persuading classmates to cheer for Jirō instead of heckling him during a relay race. He works part-time at the same café as Minami and enjoys video games, occasionally bonding with Jirō over this interest. His history prior to the series is largely unknown, though he has been friends with Minami for at least a year, with Minami confiding family conflicts in him.

His relationships center on romantic pursuits and rivalries. He is practice partners with Mei Hamano, though their rank remains low and Mei considers him a nuisance despite his occasional help in her personal matters. He maintains a friendship with Minami but envies his popularity, while Minami values their bond enough to defy parental orders to sever ties. His interactions with Sachi and Natsumi Ōhashi are strained due to his persistent focus on Akari, resulting in physical confrontations. He views Jirō as a rival but gradually develops a grudging respect, shifting from confrontation to limited camaraderie after the sports festival incident.

He is the only major character not named after a Japanese train station and debuts later than other supporting characters. His first name rarely appears in the manga, with "Syu" used as a username once, though visible in promotional materials and anime credits.