Shin Shiyou, also known as Qin Siyao, possesses a unique Nenyou constitution that enables her to heal the Youmeishi. This ability solidified her arranged engagement to Tanmoku Ki. Her appearance features mid-back length brown hair, violet eyes, and thin eyebrows visible beneath her fringe. Publicly, she plays the part of a jovial and respectful young woman, often exhibiting overt fangirl behavior around Ki Tanmoku. Privately, she embraces a contrasting identity as an avid otaku deeply immersed in video games, a world apart from her polished exterior.

Her path began in childhood when her constitution secured her role as Ki Tanmoku's fiancée. Eager to prove worthy, she endured years of rigorous training in etiquette, gymnastics, and academics. Despite her dedication, Ki remained consistently indifferent, fostering deep-seated emotional vulnerability. This neglect left her susceptible to manipulation by an evil spirit in Episode 2, where her suppressed negativity was exploited. Her unrequited affection persisted into adulthood, evolving to include protecting Ki from opportunistic suitors drawn to his status. She proposed a marriage without physical intimacy as a shield, but he declined.

Her character arc sees her transform from a love-stricken fiancée to a figure of self-sacrifice. Recognizing Ki's profound bond with his spirit shadow, You Keika, she relinquishes her claim, symbolically transferring her commitment to Keika—prioritizing his happiness over her own desires. Her relationship with Keika is complex, marked by initial jealousy over his closeness to Ki, yet she affirms his capabilities by declaring him the strongest spirit shadow. Her dialogue reveals insecurities: "He has always been out of reach for me... I am just a member of the unnamed audience" and devotion: "Pride disappears the instant you truly begin loving someone."

Beyond romance, she serves as a quietly observant and protective figure, safeguarding both Ki and Keika, reflecting her underlying kindness and resilience.