Hyakutarō serves as a guardian spirit bound to Ichitarō Ushiro, manifesting physically behind him during supernatural threats to intervene directly. His presence emerges exclusively in Ichitarō’s moments of extreme peril or anguish.

In the premiere episode, Hyakutarō materializes to rescue Ichitarō from a teacher driven to murder by a fox spirit summoned through a Kokkuri-san ritual. This intervention successfully exorcises the spirit and ensures Ichitarō’s survival.

During a later poltergeist attack in the second episode, Hyakutarō fails to appear, resulting in Ichitarō’s possession. The absence compels another psychic, Funakoshi, to perform the exorcism. Distraught, Ichitarō prays for a stronger bond and vows to live virtuously, prompting Hyakutarō’s return.

After Ichitarō’s plea, Hyakutarō unveils a previously concealed aspect of his nature by transporting Ichitarō on a guided tour of the afterlife. This journey presents an afterlife cosmology directly inspired by Emanuel Swedenborg’s spiritualist writings, shifting the narrative toward spiritual exploration.