Kaya is a young girl from the Emishi village, introduced at the very beginning of the story. She has brown hair tied with a single red bow and brown eyes, and she wears a purple kimono with a red lining. She also wears a straw hat, which is typical for her village. In her first appearance, she demonstrates notable courage when she and two other girls flee from the demon Tatari Gami. When one of her companions trips while running down a hill, Kaya draws her sword and turns to face the monstrous boar god, attempting to defend her friends even though she knows she is outmatched. Ashitaka arrives and saves them by shooting the demon in the eye.

Kaya speaks to Ashitaka using the honorific "anisama," which means "brother," though this is a term of affection and respect within the small community rather than an indication of a literal sibling relationship. Within their close-knit Emishi community, Kaya is betrothed to Ashitaka. According to Miyazaki, she was already pregnant with Ashitaka's child, and the legend of Ashitaka would be passed down through their descendants.

Her most significant moment occurs after Ashitaka is cursed by the demon. Because of this curse, he is declared dead to the village and banished forever, with tradition strictly forbidding anyone to see him off. Despite this taboo, Kaya defies tradition to say goodbye to the man she loves. In the darkness, she gives him her most precious possession: a small crystal dagger. Her words express her fear that he will forget her, and she asks him to keep the dagger as a way to remember their bond. In Emishi tradition, a girl gave a crystal dagger to her future husband on the day of their wedding. By giving him this gift as he leaves, Kaya implicitly acknowledges that this marriage can now never take place, yet she wants him to carry this symbol of her love with him on his dangerous journey.

Kaya does not appear again in the film after this farewell. However, the crystal dagger continues to carry her presence forward as a key item in the narrative. Later, when Ashitaka meets San and wants to demonstrate the depth of his feelings and his sincerity, he gives the crystal dagger to the wolf-girl. The act represents a transfer of devotion; just as Kaya meant everything to Ashitaka when he lived in the village, San has come to mean the same to him in his new life. This moment adds a layer of poignancy to the love triangle within the story, as the gift from one female character is entrusted to another.

In terms of personality, Kaya is defined by her loyalty, her quiet strength, and her lack of fear when protecting her friends. She does not appear in the film's main action at Iron Town or the forest, as her role is primarily to represent what Ashitaka has lost: a peaceful, traditional life, a loving community, and a future with a family. She embodies a gentler, more idealized world that stands in contrast to the violence and industrial conflict that defines Iron Town and the war with the forest gods. She does not possess any supernatural abilities or combat training beyond fighting with a simple sword. Instead, her power is emotional, as her farewell gift serves as a crucial link between Ashitaka's past and his future path toward a cure for his curse.