Byakko is a mysterious woman pursuing immortality who emerges as an antagonist. She is later revealed to be the "Aya" warrior serving Seiju Shinano (Masaki Hyuuga), the main antagonist and father of protagonist Mayuki Hyuuga. Byakko possesses flowing gray hair reaching her hips and distinctive red eyes. Her name, meaning Polygonatum Odoratum, aligns with the floral naming conventions of other Ayas.

Significant emotional trauma defines her past: she was deeply scarred by abandonment when her older sister, Yoko, chose to become the Aya for Shien Shinano (Seiran's brother). This rejection drove Byakko to seek purpose and power by aligning with Seiju Shinano and undergoing transformation into his Aya, cementing her role in the conflict.

As Seiju's enforcer, Byakko actively generates and manipulates supernatural "phantom cases" throughout Kyuto. These incidents test Mayuki's developing extrasensory abilities, destabilize the city, and advance Seiju's objectives. She operates with unusual autonomy for an Aya, retaining free will while executing missions. Her methods involve manipulation and direct confrontation with Mayuki and his allies.

In combat, Byakko wields abilities intrinsic to her Aya nature, using her hair as a whip-like weapon for offense and defense and implementing sharp claws. Her aggressive fighting style reflects her relentless pursuit of Seiju's goals. Her arc concludes with defeat following Seiju's ultimate failure, resolving her direct threat.