Sterling is a purebred, male airedale terrier. He has mainly dark coffee brown fur and, darker brown on his eyelids, and bushy dark grayish-brown. His tail is short and curved. He has flipped ears and are rounded at the end, at the end of his muzzle his fur is shaggy and has a black nose. He has dark gray fur that begins at his neck, close to his ear, that at the edges is wavy.
Sterling is a stern, dignified dog. He believes playing with dog toys to be beneath him, and snubs Rebound. It is soon revealed that Mildred is actually Sterling's second owner. He was given to her by his previous owner after he came in second place at one dog show. This resulted in Sterling being determined to win at all costs, for fear of being cast aside again.
Sterling is first seen arriving at Agatha's house with Mildred. He blows off Rebound's attempts to make friends with him, and acts smug when Mildred mentioned that he was entered in the dog show. When Agatha announced that she was entering Rebound in that same show, he dismissed the chances of her winning.
At the show, Sterling realised that Rebound could actually possess the skills to win. In order to break her spirit, he coldly berated Rebound, driving her to tears (He privately lamented this act right afterward, though). He also tried to sabotage Rebound's performace with a squeaky toy, which failed miserably.
In the end, Rebound won first prize, and Mildred considered giving Sterling away, to his despair. Agatha berated Mildred for this attitude, telling her that Rebound was her best friend, and that Mildred should treat Sterling in the same manner. In response, Mildred tentatively petted Sterling, then started giving him more enthusiastic affection, much to his delight.
Sterling later made an appearance (along with Mildred) in a crowd of people and their pets in "Squawk".