Originating from Mexico, Atoru was orphaned during her family's time in Texas. After praying to the Aztec god Quetzalcoatl, she inadvertently summoned a Yoi—a supernatural entity appearing as a massive horse capable of transforming into a dragon. Named Quetzl but called Yukiwa by Atoru, this creature became her companion. Inspired by tales of Japan from shipwrecked samurai, she journeyed there seeking refuge.

In Edo-era Japan, Atoru faced intense discrimination as a foreigner. To survive, she artificially lightened her bronze skin tone with makeup to pass as Japanese. She and Yukiwa performed together in a circus until her true origins were exposed. Following this, the cross-dressing Ayashi member Genbatsu arranged for her to live under the protection of geishas in the red-light district, though she did not become a geisha herself.

Atoru exhibits unique perceptive abilities, seeing the Yoi Kumoshichi—an entity previously visible only to Yukiatsu—and correctly identifying its non-human nature despite its human guise. This ability remains unexplained. Her relationship with Yukiatsu evolved into a protective, almost familial bond, with him acting as a guardian figure. She shares his status as an outsider, reflecting his empathy for marginalized individuals.

During a crisis where Yukiwa went berserk, Kumoshichi merged with the creature to calm it. This fusion granted Yukiwa Kumoshichi's sentience and voice, altering Atoru's dynamic with her companion.

In the OVA "Ayashi Divine Comedy," Atoru participates in events creating a stable time loop. She and the Ayashi travel to the past, aiding the young Yukiatsu in returning from the Other World, thereby influencing his origins.

Note: One source inaccurately describes Atoru as a 12-year-old Edo-born boy suffering from disease. All other sources consistently depict her as an Aztec girl, making this description likely erroneous.