Banryū Reoko is a character from the anime who once wielded the powerful Buranki known as Entei's Heart. Her history is deeply intertwined with that of Migiwa, to whom she was closest as a best friend during their high school years. The course of her life changed dramatically when she was informed that she had only three years remaining to live. In response to this prognosis, Reoko resolved to reveal the existence of Entei and the other Buranki to the world. However, Migiwa intervened in this plan, believing it to be an attempt to save Reoko. Using the Buranki Oubu, Migiwa destroyed Entei's limbs and crushed its Heart. This act caused a new Core to manifest painfully within Reoko's own body, transforming her into Entei's new living Heart and, as a consequence, granting her immortality.

Following this transformation, Reoko was imprisoned within Migiwa's mansion for an extended period. She was eventually released upon Migiwa's departure for Treasure Island. Despite the passage of time, Reoko has not aged physically, remaining unchanged even as she reaches the age of forty. Upon her release, she reunites with her former comrades. Together, they overthrow a government that had been systematically killing Bubuki-users and establish a new regime that functions as a puppet government under Reoko's control. This entire political maneuver serves her ultimate, personal objective: to exact revenge against Migiwa for the actions that fundamentally altered her existence.

As the living Heart of Entei, Banryū Reoko possesses abilities that are similar to those of Azuma. However, this power comes at a severe and tragic cost. Every time she controls Entei, it inflicts fatal damage upon her own body. While her immortal body is capable of reviving and restoring itself to near-perfect condition after death, the damage to her brain is permanent and cannot be recovered. Consequently, each time she dies and is resurrected, her memories are progressively eroded and eaten away, a slow and insidious form of degradation that defines her character's tragic condition. Her personality and motivations are driven by this history of betrayal and suffering, with her actions in the story largely centered on her long-nurtured plan for revenge against her former best friend.