Setsuna Yūki, born Nana Nakagawa, is a second-year student and the first sophomore to hold the title of student council president at Nijigasaki High School. Her scarlet-red image color mirrors her vibrant persona. Standing 154 cm tall with a slim build, she sports long bluish-dark gray hair styled in a right-side ponytail adorned with a rose-shaped ornament, complemented by pale gray eyes. As Nana, she swaps this look for glasses and twin low pigtail braids.

A transformative encounter with school idols ignited her passion for the craft, leading her to adopt the alias "Setsuna Yūki" to conceal her activities from her strict family. She founded Nijigasaki’s original idol club, but conflicting visions—particularly with member Kasumi Nakasu—caused its dissolution. Undeterred, Setsuna continued performing solo, her dynamic stage presence sparking rumors of her identity as an enigmatic "phantom idol" among peers.

Balancing dual roles, Nana excels academically, diligently completing assignments and studying during downtime. She supports friends by coaching dance routines, sharing vocal tips, and championing their ambitions. Her charisma shines through an energetic smile and lively performances, fueled by fervor for anime, manga, gaming, and idol culture. Though driven, she remains attentive to others’ needs.

As part of the subunit A・ZU・NA with Ayumu Uehara and Shizuku Osaka, Setsuna memorizes every student’s name at Nijigasaki. The Rainbow Days Honest Photo Essay reveals she wears contact lenses during idol appearances, and her classic idol suit, though featured in promotional art and anime openings, never appears in episodes. She occasionally contributes to Love Live! Days’ No Game, No Life!! column, discussing titles like Call of Duty: Warzone, Octopath Traveler, Street Fighter V: Champion Edition, Romance of the Three Kingdoms 14, and Sekiro: Shadows Die Twice. Her stage name draws inspiration from a light novel character cherished by Nana.