Arthur Shelby is the first villain that the boys encounter who has a love for gadgets. Upon arriving at his home for the first time in Seaside, they encounter a handful of gadgets used by Shelby as pranks to discourage visitors, and Shelby lets them know immediately that he is a prankster by using a false hand to shake with Jupiter as the boys leave his home.
Like Rawley in "Vanishing Treasure," Shelby has an elaborate plot to rob a bank via a tunnel. Being a former engineer in the city, he knows of several unused tunnels underneath the town, one of which goes underneath a bank. His construction of the dragon is to help him in the robbery attempt.
Of course, Shelby hadn't counted on the boys' investigative abilities. When the boys ruin his attempt at the robbery, Shelby reveals his true nature as a prankster rather than a criminal by letting the boys go and returning the gold to the bank. Definitely not the typical criminal.