The Black Cat, an apparition capable of appearing or vanishing at will, shifts effortlessly between corporeal and incorporeal states. It frequents the presence of Ouri Kagami, communicating exclusively with him. During events tied to Shikabane or Shikabane Hime, the cat emerges to offer Ouri cryptic clues about these entities. While its motivations and allegiances are shrouded in uncertainty for much of the narrative, the cat occasionally becomes visible to Makina Hoshimura and Akasha Shishido.

This spectral entity mirrors the exact appearance of Ouri's cherished childhood pet cat, which perished in a vehicle accident. The Black Cat manifested shortly after the pet's death. Approaching the series' end, Hizuchi—a former orphanage companion of Ouri—helps unveil the cat's true nature: a collective manifestation formed from multiple children kidnapped by Ouri's mother. Driven by her attachment to the living world and a desperate desire to hold her child, these deceased children transformed into Shiryou (restless fallen spirits) and merged into a singular feline form.

In the climactic confrontation, the cat engulfs Ouri, morphing into a monstrous entity. Despite this violent act, the cat's actions ultimately aid Ouri before vanishing without a trace. Throughout the story, the Black Cat consistently steers Ouri toward supernatural encounters, especially those involving Shikabane, though whether its guidance is benevolent or manipulative remains ambiguous. The cat's origin as a conglomerate of lost spirits intertwines with the series' broader themes of regret and unresolved obsessions surrounding the undead.