Originally known as Hai-Ryūn Ieasomūru Jeda, Hanyuu was an ancient demon goddess who led her clan to the Onigafuchi swamp seeking human coexistence. Her distinctive purple horns caused discrimination, yet Furude Riku fell in love with her, nicknamed her "Hanyuu," and married her. Their daughter Ouka began a lineage blending demon and human blood, which became the origin of Hinamizawa Syndrome—a condition causing paranoia and violence in advanced stages.

To mediate peace, Hanyū pretended to sow discord, offering to bear the villagers' sins. This resulted in her capture and sacrifice during the first Watanagashi Festival. Manipulated by the village priest, Ouka struck Hanyū with the sword Onigari-no-ryuuou, cracking one horn and killing her. After death, Hanyū became revered as the guardian deity Oyashiro-sama, existing incorporeally for centuries, unable to interact with humans.

A millennium later, Rika Furude, Ouka's descendant and eighth Furude heir, was born as "Oyashiro-sama's reincarnation." Rika could perceive and interact with Hanyū, who became her sole companion and maternal figure, teaching her essential skills. Upon Rika's deaths, Hanyū gained the ability to travel between Fragments (parallel timelines), looping Rika back two weeks before each demise to alter fate. Hanyū's cracked horn diminished her powers over time, limiting memory retention between Fragments.

Incorporeal Hanyū often followed villagers afflicted by advanced Hinamizawa Syndrome, apologizing for her inability to prevent their suffering. Her unseen presence—manifesting as footsteps, whispers, or a feeling of being watched—exacerbated their paranoia, contributing to Oyashiro-sama's curse legends. She occasionally assumed a visible human form resembling Rika's age, wearing shrine maiden attire or school uniforms.

Hanyū was timid and melancholic, prone to muttering "au" when nervous or emotional. She expressed guilt over historical events and looping tragedies, frequently advising Rika against excessive hope to shield her from disappointment. Despite pessimism, she valued her bond with Rika above all, stating she would endure centuries of loops to remain with her. In rare moments of resolve, she demonstrated manipulative or assertive traits, using her goddess form to intimidate adversaries like the Mountain Dogs or Miyo Takano.

Her abilities included fragment travel, time looping, time-stopping, teleportation, flight, mind control, and briefly curing Hinamizawa Syndrome. Activating divine powers turned her pupils red or purple. She shared a sensory link with Rika, experiencing her physical sensations. In later events, she wielded Onigari-no-ryuuou proficiently and harnessed Fragments to empower herself against adversaries.

In *Higurashi Gou* and *Sotsu*, the higher-dimensional entity Eua identified Hanyū as her "failure" and a splintered fragment created when Eua's horn was damaged. Audio commentaries confirmed Hanyū originated from a "small Eua" split-off. Eua implied Hanyū's existence was tied to an imperfect looping mechanism caused by the horn crack. During confrontations, Hanyū temporarily regained strength to challenge Eua, damaging her horn before departing the Fragments.

Key relationships included her maternal devotion to Rika (enduring occasional teasing) and tentative kinship with spiritually sensitive Rena Ryūgū. Her dynamic with Eua remained adversarial yet intrinsically connected.

Hanyū's story concluded with her ascending from the Fragments after Rika achieved lasting peace, though her legacy influenced subsequent conflicts between Rika and Satoko.