Agent Arthur is an agent with the Action Agency.
We learn very little about this organization. We are told that it "is a world wide undercover organization dedicated to fighting crime and solving mysteries". Further, "supremely successful, the Agency lives up to its motto, "Search, Solve and Survive", by operating a 'go anywhere, do anything' service".
Arthur (no last name given) is the newest recruit to the Agency. He is also the youngest they have had. While he got the job because he is highly intelligent and fiercely determined, it probably did not hurt that his uncle, Jake Sharpe, a very mysterious man who is rarely seen in person, was the founder of the Agency and the main brains behind the organization.
Arthur is shown to be a teenage boy, likely around 14-15 years of age with reddish-blond hair. Tall and quite lanky, he has a long, narrow face which does not bother trying to hide his emotions.
As his only traveling companion (usually), Arthur has a dog named Sleuth who possesses quite a few similarities to Arthur in that he is not only very smart and totally loyal, he is long and lanky.
Despite being a teenager who could not legally drive a car or fly a plane all alone, this young fellow is sent to all sorts of places around the world, handling mysterious situations on his own. But being the very intelligent and highly self-sufficient, he obviously can handle these dangerous missions and actually looks forward to the next one. And he is able to provide entertaining puzzles for us to help solve along the way.