Youko is the Aya—a puppet warrior—and secretary serving Shien Shinano, headmaster of Mayuki Hyuuga's school. She assists Shien in his duties while maintaining order, frequently using a harisen to discipline his childish or irritating behavior. Her younger sister Byakko serves as an Aya to Seiju Shinano (Masaki Hyuuga). Their relationship remains strained due to Youko abandoning Byakko in their youth to become Shien's Aya, an act that fueled Byakko's depression and eventual alignment with Seiju as his doll.

As Shien's Aya, Youko operates as his bodyguard and combatant, mirroring roles held by other Ayas like Hatsumi/Souka and Sanae/Yuuhi under Seiran. Her specific fighting style or abilities are not detailed. She consistently fulfills her duties supporting Shien and interacting within the school environment, while her estrangement from Byakko persists throughout the series.