Erin originates from Ake Village in the Tai-Koh Region, raised by her mother Soyon, a skilled Touda caretaker. By age 10, she exhibits intense fascination with animals, especially the dragon-like Touda bred for war. Her green hair and eyes, inherited from her mother's Ahlyo heritage (the Mist People), mark her as mixed-race, leading to discrimination and the slur "Akun Meh Chai" (devil-bitten child). The Ahlyo lineage carries stigma due to rumors of forbidden beast-control techniques.

After the mysterious deaths of prized Kiba Touda under Soyon's care, authorities blame Soyon. She suffers the "Judgment of the Touda," executed by being devoured by wild Touda while Erin watches. During the ordeal, Soyon uses a forbidden whistle technique to calm the Touda momentarily, enabling one to carry Erin to safety. Erin drifts unconscious downriver, washing ashore in the Shin-Oh Region.

Found by the beekeeper John, he adopts her. John teaches Erin beekeeping, reading, and crafting musical instruments like harps. Recognizing her intellect and independence, he supports her pursuit of education despite societal expectations for girls to marry young. During this time, she observes wild Royal Beasts (Ohju), majestic symbols of Shin-Oh's divine rule, sparking deep fascination.

At 14, Erin enrolls at Kazalumu Beast Sanctuary to train as a beast doctor. She encounters Leelan, a critically injured Royal Beast cub refusing food. Defying protocols, Erin insists on caring for Leelan, rejecting traditional methods like the paralyzing Silent Whistle and strength-enhancing tokujisui potion. Instead, she uses her harp to mimic natural beast sounds, successfully coaxing Leelan to eat. She raises Leelan with minimal human interference, fostering an unprecedented bond where Leelan views Erin as a mother. This approach allows Leelan to regain abilities lost in captivity, including flight.

Leelan later mates with Eku, a wild Royal Beast brought to Kazalumu injured, producing the first captive-born Royal Beast cub, Alu. Erin graduates top of her class and becomes a Kazalumu teacher to stay near Leelan. Her unique bond attracts political scrutiny. Duke Damiya, the Shin-Oh's nephew, attempts to coerce her into training Royal Beasts for war, threatening others if she refuses. An Ahlyo elder also warns her that communicating with beasts is a "mortal sin," hinting at future conflict.

By 18, Erin becomes entangled in escalating tensions between Shin-Oh's territory and the militaristic Aluhan's territory. When the elderly Shin-Oh, Halumiya, is ambushed by rogue Touda, Erin rides Leelan into battle to protect her. Leelan kills the attackers, but Halumiya later succumbs to injuries. Damiya frames the Aluhan for the assassination. Erin deduces Damiya's treachery and uncovers the kingdoms' founding truth: centuries prior, a priestess named Jeh allied with the Aluhan's ancestor, using Touda and Royal Beasts to destroy the kingdom of Ofahlon. Horrified, Jeh established the Royal Beast Canon to prevent such control and became the first Shin-Oh.

Erin shares this history with the new Shin-Oh, Seimiya, exposing Damiya's role in Halumiya's death. As war erupts between the Aluhan's sons (Shunan and Nugan) and Damiya's forces, Erin reluctantly intervenes. She sends Leelan to rescue Shunan on the battlefield, sacrificing her own safety. An arrow strikes Erin during the act. Leelan retrieves her and flees. Post-conflict, Erin survives and is implied to form a family with Ialu, a Shin-Oh bodyguard; they have a son named Jesh. Her life's work centers on fostering mutual understanding between humans and beasts, rejecting their use as weapons.