Osaki-ko, known as O-chan, is a snowy-white fox spirit (Osaki) with blue eyes, two tails, and a distinct blue spirit mark on its forehead. Serving as a guardian and guide within the Tamayori Princess lineage, its gender remains ambiguous despite the current princess, Tamaki Kasuga, initially perceiving it as female.

This spirit forms an immediate bond with Tamaki upon her arrival in Kifumura, becoming her constant companion. Exhibiting a playful and curious nature, Osaki-ko communicates through mewing sounds and notably guides Tamaki into the forest during their first encounter. This guidance proves vital during Tamaki's initial confrontation with hostile entities.

As the Tamayori Princess's protector, Osaki-ko possesses spiritual abilities. It can travel within Tamaki's shadow, rendering itself invisible, and shares its senses, allowing Tamaki to see and hear through the spirit. During critical confrontations with dark forces, Osaki-ko merges its power with Tamaki, transforming into a translucent blue circlet upon her brow. This fusion initiates the sacred robes and full powers of the Tamayori Princess, enhancing her ability to seal artifacts like Onikirimaru and create protective barriers.

Beyond its primary bond with Tamaki, Osaki-ko also displays a particular connection to Yuuichi Komura, one of the five Guardians. Its role is intrinsically tied to the Tamayori Princess's duties, acting as both protector and catalyst for accessing her latent powers against supernatural threats.