Kōki Nanjō is a student at Ayanagi Academy and a member of Team Sazanami. He is self-described as a versatile and cunning type, capable of performing any task reasonably well if he puts in a moderate amount of effort. This belief leads him to approach most activities without deep passion or enthusiasm, maintaining a relaxed and nonchalant demeanor. His tendency to coast by on natural talent rather than dedicated hard work has made him the only person whom Ageha Riku openly declares he dislikes, though Nanjō himself is unbothered by this. People meeting him for the first time often find his presence unsettling or slightly intimidating, a perception reinforced by his habit of keeping a calm smile while making sharp, sometimes cutting remarks. He carries a belief that everyone possesses a certain duality and is cautious about revealing his own weaknesses to those he does not yet trust.

During his middle school years, Nanjō served as vice president of the student council, and he maintains a connection with an acquaintance from that time who is now a member of the Kao Council. This background hints at a capacity for responsibility and leadership that he seldom puts on display in his daily conduct. Within the story, Nanjō is part of the cohort of second-year students competing for the elite training slots under the guidance of the Ancient mentors. His role is primarily as a supporting figure within his team, where his lack of overt ambition contrasts with the intense drive of many of his peers. His relationship with his teammate Kitahara Ren is notable, as both share a similar attitude of high potential tempered by a reluctance to fully commit to grueling effort. Over the course of the series, Nanjō’s interactions with others and the pressures of the audition process gradually push him to confront his own apathy. While detailed personal growth is limited in available information, he is shown to possess the underlying ability to perform at a high level when circumstances demand it. His notable abilities include a versatile skill set in singing and acting, built on natural aptitude and a quick grasp of techniques, as well as the social acumen developed during his student council days.