Soyon, an Ahlyo tribeswoman identified by green eyes and mystical associations that fostered societal prejudice, resided in Ake Village after marrying Asson, the chief's son. Following Asson's early death, she raised their daughter Erin alone while serving as the head veterinarian for the Touda—amphibious reptiles used as military assets by the Grand Duke. Soyon possessed exceptional expertise in Touda biology and behavior, making her indispensable despite persistent discrimination due to her heritage. She cautiously shared veterinary knowledge with Erin, encouraging her daughter's curiosity about animals, but deliberately withheld the Ahlyo's forbidden techniques for communicating with or controlling beasts.

A pivotal event occurred when the Kiba, the strongest Touda under Soyon's care, mysteriously died overnight. Village authorities held her responsible and sentenced her to execution by wild Touda. During the execution, Soyon performed a forbidden act: using a whistle to momentarily calm the wild Touda, violating Ahlyo oaths in a technique known as "Souja no Waza." She sacrificed herself to the creatures, placing Erin onto a Touda's back to ensure her escape.

Ahlyo elders later confirmed Soyon deliberately caused the Kiba's death. She exploited their breeding season vulnerability by contaminating their water with "Tokujisui," a special potion that acted as poison during this sensitive period. This act aimed to prevent the Kiba's use in war, aligning with her ethical opposition to exploiting beasts.

Soyon's influence profoundly shaped Erin's values, instilling empathy for animals, a commitment to ethical treatment, and the importance of critical thinking. Her death and the revelations surrounding her choices became a driving force in Erin's journey to understand the relationship between humans and beasts.