Ratso, a black-and-white neighborhood cat, regularly crosses paths with the large white dog protagonist as his rival, though their dynamic occasionally shifts to fleeting camaraderie. His behavior brims with annoyance and mischief, routinely disrupting the dog's daily life and causing general chaos.

A recurring tactic sees Ratso feigning friendliness or assistance while covertly sabotaging the eccentric inventions of the dog's owner, a professor. This interference typically triggers malfunctions or backfires, escalating disorder.

As a primary source of conflict, Ratso thwarts the dog's quests for peaceful invention testing or tranquil days. His motivations stem from troublemaking and personal amusement rather than deep-seated malice.

Internationally, the character adopts varied aliases: Sénéchal (French), Blackie (Japanese/Polish), Balthazar (Dutch/Spanish), Gatto (Italian), and Mushakess (Arabic). All iterations preserve his core identity as an annoying cat who disrupts the inventor's work and the dog's routine.