Osushi is a Disguiser Dragon inhabiting the reanimated body of a deceased Pomeranian dog discovered by Balgo Parks on London's streets. Balgo cared for this entity for two months before its true nature surfaced, naming it Osushi.

In its canine disguise, Osushi appears as a small Pomeranian with predominantly white fluffy fur, accented by brown coloring on its muzzle, ears, and paws—an intentionally unremarkable facade. Transforming into its true Dark Dragon state reveals a large, heavily scaled creature with black coloration, a lighter underbelly, four thick legs, two expansive wings, prominent sharp teeth, and significant physical strength.

Behaviorally, Osushi exhibits docility and compliance in its dog form, obediently following Balgo's instructions like acting as bait to attract Noel Niihashi or distracting others through perceived cuteness. Transformation into a Dark Dragon triggers intense aggression, including attempts to consume Balgo and Shelby, fueled by Dark Dragons absorbing human negative emotions and developing a predatory drive toward humans. Osushi periodically demands to see Noel's panties, a phrase learned from Balgo but uttered without apparent comprehension.

Osushi displays protective instincts toward Balgo. Agitation and partial transformations—typically manifesting as large black wings while mostly retaining canine form—occur in response to Balgo imitating the dog, sneezing, laughing loudly, or facing danger. Physical contact like holding and petting can calm Osushi and reverse the transformation, indicating an underlying connection beyond simple cohabitation.

Key events begin with Balgo using Osushi to encounter Noel, accidentally exposing it to the supernatural realm. Later, accompanying Balgo and Shelby to investigate Noel's disappearance, Osushi fully transforms into a Dark Dragon and attacks both companions until confronted by Noel and Ninny Spangcole. Osushi's repetition of the "panties" demand alerts Noel to Balgo's proximity; Noel subdues and reverts Osushi to its dog form. Following Balgo's designation as a Dragon-Possessed One, both relocate to Reverse London under Noel and Ninny's supervision. There, Osushi continues partial transformations during walks, causing property damage by creating holes in buildings. Investigation reveals these transformations function as an early warning system triggered by sensed danger. A specialized Fence Collar manages these uncontrolled changes.

Osushi subsequently accompanies Balgo during activities including residing at Noel's home, investigating illegally-reared Dragons, evading detention attempts by the Inks, and observing interactions between Balgo and Noel. During one incident involving Balgo's unconscious manifestation of an energy sword, Osushi licks his neck, maintaining physical proximity.

Capabilities include limited human speech—specifically the phrase about Noel's panties—attributed to absorbing Balgo's negative emotions. Partial transformation allows wing manifestation while predominantly retaining canine form, granting flight capacity and sufficient strength to carry Balgo. Full transformation yields greater size, physical power, and aggression, reversible through calming techniques.