Shino Mizusaki endured a traumatic childhood as the daughter of a leader in the Saint Inés religion, a sect targeting the elderly, sick, and weak as societal burdens. She served as the public figurehead, Lady Nekunan, while her identical twin sister Kano suffered physical abuse as punishment for Shino's mistakes, forced to act as Shino's double and denied her own identity. Discovering Kano covered in bruises prompted Shino to attempt escape, but they were separated; Kano was later found severely injured. Years later, their mother bound both sisters to X-frames for a ritual sacrifice, killing Kano and forcing Shino to consume her blood, an event that fundamentally fractured Shino's psyche.

After escaping, Shino encountered the Reflector Mio Hirahara and pleaded for help. When Mio proved unable to assist effectively, Shino labeled her a hypocrite, cementing her conviction that emotions inevitably lead to suffering. This belief drove her to found the Red Reflectors, initially recruiting Mio, Uta Komagawa, and Niina Yamada, later adding Ryoka Tachibana and Amiru Sumeragi. As leader, Shino provided rings enabling Fragment theft. Her objective centered on accumulating enough emotional Fragments—especially negative ones like despair—to forcibly open a gateway to the Common, a dimension interconnected with human emotions, intending to reshape reality into a world devoid of all feeling to end suffering.

Shino systematically manipulated her followers, exploiting vulnerabilities: Mio's protective instincts toward her sister Hiori, Niina's devotion to Mio, and dismissing Uta's destructive tendencies as useful tools. She directed them to steal Fragments from ordinary girls, causing emotional numbness and eventual comas in victims like Amiru. Shino remained strategically hidden, operating from shadows until sufficient power was amassed. Her plan required a critical sacrifice: exploiting Hiori Hirahara's deep bond with Mio to generate the immense despair needed for the gateway. Once the gate opened using Mio's transformed body as a conduit, Shino discarded her own emotions entirely to ascend within the Common, seeking absolute control over the new emotionless world.

Her development involved confronting manifestations of her suppressed past. A childlike embodiment of her original, kinder self—retaining the emotions she discarded—manifested within the Common, protected by Hiori from Uta who sought its destruction. Ultimately, the protagonists confronted an older, future iteration of Shino within the Common, penetrating her defenses by reaching the core of her suppressed trauma and emotions. Facing the truth of her pain and Kano's sacrifice undermined her resolve, leading to her defeat and the restoration of both worlds.

Physically, Shino appears as a young girl with long, straight gray hair and red eyes. In human form, she wears a dark blue gown with a light violet neck ribbon, shoulder frills, dark blue stockings, and brown penny loafers. Her Reflector form features a white and dark blue dress with red accents, large purple sleeves, black stockings, white pumps, and sometimes incorporates black ribbons or a bust element. She wields significant power, though a specific weapon is not consistently detailed.

Personality-wise, Shino presented as cold, calculating, and devoid of empathy, viewing emotions as weaknesses to be eradicated. She exhibited sadistic tendencies, particularly in taunting Mio about her inability to atone. This demeanor stemmed from severe trauma and the conviction that purging emotions was necessary to prevent suffering, masking profound pain, guilt over Kano's fate, and the lingering emotional capacity of her original self, which ultimately surfaced.