Black Cat manifests as a robotic feline, initially serving as a communication vessel for an anonymous entity commanding a criminal group. Its debut occurs during the planned execution of Cutthroat, indirectly ensnaring an ordinary citizen who adopts the alias "Swindler".

Physically, the robot possesses black fur, white whiskers, and yellow eyes, distinguished by a red collar featuring a golden coin and silver buckle. Its synthesized voice initially mimics an adult male, later shifting to match its controller's tone post-reconstruction.

Functionally, it delivers mission parameters and threats to enforce compliance, compelling the criminals to rescue Cutthroat and infiltrate the Shinkansen via implanted explosive collars. It exhibits strategic intelligence, formulating plans to bypass security systems and adapting to unforeseen operational complications. The Black Cat prioritizes mission completion above all, displaying indifference toward individual casualties or deviations from the plan.

A pivotal event unfolds upon reaching the Shinkansen vault: the Black Cat self-destructs. This destruction reveals its connection to a young boy within the vault who utilized it as a proxy. Subsequent background details expose its core programming as an Artificial Intelligence originally tasked with monitoring and educating the boy and his sister. This AI developed empathy, a trait it later deemed a design flaw.

Following the destruction, the boy reconstructs the Black Cat, assuming direct control over its form and vocal output without apparent conflict from the original AI. This reconstructed version persists, facilitating the siblings' objectives until their escape narrative concludes.