Cian Reikadou is one of three teenage friends—alongside Tuan Kirie and Miach Mihie—who form a close bond. Together, they make a pact to rebel against their society's enforced utopia, governed by the WatchMe nanotechnology system. Administered by global health authorities, WatchMe regulates human health, emotions, and social interactions, eliminating pain, illness, and individual distress by synchronizing collective well-being, thereby suppressing personal autonomy and authentic experiences. Believing this system erases true freedom and individuality, the trio stages a radical act of defiance: a suicide pact through starvation, aiming to reclaim their sense of self outside the imposed harmony. Their resistance symbolizes a rejection of the world's superficial perfection.

During the pact's execution, Cian withdraws and alerts authorities, leading to medical intervention. This prevents Tuan's death but fails to save Miach, who completes the suicide. Cian's abandonment of the pact creates a lasting fracture with Tuan, who survives and later leaves Japan. Flashbacks suggest Cian may have perceived herself as an outsider within the trio, possibly viewing Tuan and Miach's bond as closer than her own connection to them, adding complexity to her choice.

Thirteen years later, Cian lives as an adult under the activated WatchMe system, which mandates conformity at age eighteen. Her life shifts suddenly upon reuniting with Tuan after his return to Japan. Shortly after this meeting, Cian dies by suicide. Her death occurs amid a wave of unexplained global suicides, prompting an investigation led by Tuan. This event catalyzes a central narrative involving a terrorist group developing "Harmony," a modified version of WatchMe designed to enforce global collective consciousness at a lethal cost. Cian's suicide becomes pivotal in exposing the flaws of the established system and the dangers of the new weapon. Her background as a former rebel against the system underscores the thematic tension of her fate.