Hanyuu, originally named Hai-Ryūn Ieasomūru Jeda, is a goddess worshipped as Oyashiro-sama in Hinamizawa. She possesses purple eyes, long light purple hair, and dark purple horns—one fractured. Her typical attire is shrine maiden garb with a red bow and shoulder-exposing sleeves, though she dons school uniforms or casual clothing situationally. Activating her powers turns her pupils red, signaling emotional intensity or divine action.

Centuries prior, Hanyuu and her clan settled in Onigafuchi Swamp (later Hinamizawa) seeking human coexistence. Discrimination over her horns ensued, but she bonded with Furude Riku, shrine heir, who renamed her "Hanyuu." They married, bearing daughter Furude Ōka. To shield villagers from disease, Hanyuu created a cure, attracting political greed that led to Ōka’s capture. Consumed by despair, Hanyuu slaughtered villagers. Ōka halted her rampage by cracking Hanyuu’s horn with the sacred sword Onigari-no-ryuuou, restoring sanity. Hanyuu then begged Ōka to kill her, preventing future corruption—this became Hinamizawa’s first Watanagashi sacrifice. Villagers later revered Ōka as a demon-slayer, birthing the Oyashiro-sama legend. The incident spawned Hinamizawa Syndrome: a hereditary condition inducing paranoia and hallucinations from mingled human-demon blood.

Post-death, Hanyuu lingered as an intangible spirit, unable to interact with humans for centuries. Her presence manifested through phantom footsteps and apologies, perceived by Hinamizawa Syndrome sufferers (Level 3+) as supernatural stalking that worsened paranoia. In 1983, Furude Rika became the first in ages to see and converse with her, forging a profound bond. Hanyuu served as Rika’s guardian and mother figure, teaching skills like cooking. Their senses intertwined, sharing physical sensations.

Hanyuu granted Rika time-looping powers upon death to evade June 1983’s tragedy. Yet Hanyuu’s waning strength confined loops to two weeks pre-death and fragmented Rika’s memories—later tied to her cracked horn. Hanyuu often voiced pessimism about escaping fate, urging Rika to abandon hope to preserve sanity, though she privately treasured their extended time. She carried guilt for failing to prevent tragedies, frequently apologizing to descending victims.

In *Higurashi Gou* and *Sotsu*, Hanyuu’s presence faded further. She rarely appeared in Rika’s new loops, her connection to the Sea of Fragments weakening. A turning point came when she confronted Eua, a horned entity claiming to be the "original" who labeled Hanyuu her "failure" and fragmented aspect—implying Hanyuu’s creation from Eua’s damaged horn. During their clash, Hanyuu wielded Onigari-no-ryuuou and empowered herself with fragments to fracture Eua’s horn. Eua’s gift of full-memory looping to Satoko Houjou directly sabotaged Hanyuu’s efforts, as Satoko manipulated events to trap Rika in cycles of anguish.

Hanyuu’s personality blends timidity, politeness, and distressed/excited "au au" utterances. She used "boku" as a pronoun and ended sentences with "nano desu." Despite gentleness, she displayed stern goddess-like authority when angered, especially protecting others. In *Matsuribayashi-hen*, she aided allies by intimidating Yamainu forces with a "Demonic Voice" and halted Miyo Takano by offering divine forgiveness. She also employed manipulative tactics, leveraging her appearance to sway Detective Ooishi.

Her core relationship was with Rika, whom she loved maternally yet endured verbal abuse from due to their sensory link. Rena Ryugu occasionally sensed her, likely from prior spiritual exposure. Hanyuu mourned Hinamizawa Syndrome victims like Keiichi Maebara and Satoshi Houjou, whose hallucinations her presence worsened.