Rei Asaka, alias "Saint-Juste of the Flowers," is a second-year Seiran Academy student and one of the "Magnificent Three," famed for her artistic talents and social detachment. She masters piano, flute, and violin, while also engaging in acting and basketball. Masculine traits in her character design shape her voice and demeanor.

Her background centers on a deeply destructive, obsessive relationship with Fukiko Ichinomiya, president of the school's Sorority. This bond fuels significant emotional turmoil, manifesting as depression, drug addiction, and a fascination with mortality. Despite unwavering loyalty to Fukiko, Rei exhibits protectiveness toward protagonist Nanako Misonoo, though her primary relationship hinders consistent friendships.

A pivotal development occurs through Rei's dynamic with best friend Kaoru Orihara; they exchange playful insults yet offer mutual support, creating a stark contrast to Rei's fraught connection with Fukiko. Rei's narrative arc concludes with her death: the manga depicts suicide via pill overdose, while the anime portrays an accidental fall from a commuter bridge as she attempts to catch flowers meant for Nanako.

Her demise profoundly impacts others, triggering suicidal feelings in Kaoru and instilling lasting guilt within the Ichinomiya family. Throughout the series, Rei's internal conflicts and relationships drive key plot elements, particularly concerning Nanako's journey and the Sorority's influence.