Centuries-old entity Hanyū, originally known as Hai-Ryūn Ieasomūru Jeda, began as a demon goddess settling in the swamp that became Hinamizawa. Her attempts to coexist with humans sparked conflict due to her horned appearance. She married Furude Riku and bore a daughter, Ouka. When disease spread, Hanyū created a cure, but political greed led to Ouka's capture. Consumed by rage, Hanyū massacred villagers before Ouka subdued her using the sacred sword Onigari-no-ryuuou, cracking one of Hanyū's horns. Hanyū then requested her own death to prevent future corruption, resulting in her sacrifice and deification as Oyashiro-sama.

After death, Hanyū existed as an incorporeal spirit for over a millennium, silently observing Hinamizawa. She gained the ability to interact with her descendant, Rika Furude, the first person to perceive her in centuries. Hanyū acted as a maternal figure, teaching Rika skills like cooking. Their senses became intrinsically linked, meaning Hanyū experiences Rika's physical sensations, including pain or taste. When Rika began dying repeatedly in June 1983, Hanyū used her powers to transport them both to new fragments of reality, allowing Rika to relive events to alter fate. Over time, Hanyū's abilities diminished, restricting resets to two weeks before Rika's death. The crack in her horn also caused Rika's incomplete memory retention between fragments.

Hanyū's personality contrasts with her divine status; she is typically timid, prone to anxious vocalizations like "au au," and quick to apologize. She expresses pessimism about escaping fate, often advising Rika against excessive hope to shield her from repeated disappointment. Despite this, she cherishes her bond with Rika, stating she would endure centuries more loops to remain by her side. Her guilt over past events manifests as compulsive apologies to villagers affected by Hinamizawa Syndrome, inadvertently worsening their paranoia—a behavior revealed as the source of the "mysterious footsteps." In rare moments of anger or divine intervention, her demeanor shifts: her eyes glow red, her voice becomes authoritative, and she wields powers like time manipulation, teleportation, or mind control. She notably confronts antagonists like Miyo Takano, offering forgiveness as a deity while emphasizing her separation from humanity.

In *Rei*'s "Dice Killing" arc, Hanyū orchestrates an elaborate dream world after Rika sustains life-threatening injuries in a bicycle accident. This world diverges from their reality, featuring idealized lives for Rika's friends and the presence of Rika's living parents. Hanyū remains excluded from this fragment, communicating only intermittently. Her intention is therapeutic: forcing Rika to confront her emotional detachment from her parents after centuries of grief and to appreciate the value of her original world, where friendships were strengthened through shared trauma. The arc culminates in Rika facing an illusory choice to "return" by destroying a fragment within her dream-world mother—a manifestation of Hanyū's lesson about loss and gratitude. Though Rika awakens from her coma without committing the act, the experience reshapes her perspective on her relationships and sacrifices.

Later continuations reveal Hanyū's origins as a fragment of a higher entity named Eua, who labels Hanyū her "failure" due to the horn damage. This connection grants Hanyū the ability to harness fragments as weapons, enabling her to defeat Eua in a climactic battle. Her role expands beyond guiding Rika to confronting existential threats tied to her own fractured existence.