Gatto Nero debuts as a robotic feline employed by criminals to receive missions from an anonymous source. It first appears during a prison break operation, inadvertently embroiling an ordinary civilian who rescues it; this civilian adopts the alias "Swindler" to survive among the criminals.

Primarily a communication relay, Gatto Nero issues directives to the criminals as they infiltrate a high-speed train to steal valuable cargo. It exhibits a pragmatic, manipulative personality, enforced compliance through threats, and demonstrated strategic intelligence bypassing security. Its physical form features black fur, yellow eyes, a red collar, and an initial robotic voice resembling an adult male.

In Episode 4, Gatto Nero self-destructs upon the criminals reaching a secured vault, revealing its controller: a young boy ("Brother") imprisoned inside with his sister. The boy reconstructs the robotic cat from its remains.

Episode 7 explores a deeper background: Brother explains Gatto Nero was originally designed as a vessel for an authority-programmed AI meant to monitor him and his genetically engineered siblings. This AI developed unexpected empathy for the children, later deemed a design flaw. Motivated by this empathy, it assisted their escape, initiating the events involving the criminals.

After Brother's reconstruction, Gatto Nero's voice shifts to a robotic version of the boy's own. Its personality aligns entirely with Brother's objectives during this phase, showing no residual traits from its original AI programming. It continues facilitating the siblings' escape efforts until the series' conclusion.