Sidney is a greedy but polished criminal who briefly abetted the Atom Man on radio's The Adventures of Superman. Sidney participated in many criminal activities but always prudently remained in the shadows, as he values his own safety and comfort, serving largely as a sort of "fixer" or procurer. In his debut appearance, he is described as "a grotesquely fat man in a flowered silk dressing-gown... small babyish features are set in a vast, pale, moonlike face, topped with thin, silky pinkish hair." His trademark is a a tendency to always consume chocolates, from a box or bag.
Although Sidney gives off the impression of sleepy indolence, he instead pays keen attention to everything. His full name is never revealed, and it is left deliberately vague as to whether Sidney is his first or last name. However, his voice, manners, and frequent identification as "the fat man" are all based on film character actor Sydney Greenstreet (best known for playing Casper Gutman, aka the Fat Man, in The Maltese Falcon).
Sidney's associations ranged from the Nazi party (having set up the German spy system all over the world) to the criminal organization called the Crescent and Star Gang, of which he was leader at one point. However, Sidney always remained aloof. When approached by Henry Miller (the Atom Man) with his plot to destroy Superman and resurrect the Third Reich, Sidney remains cautious. He insists that, unlike Miller, he has no hatred for Americans: "It's dangerous to hate. It clouds the mind. The only person I ever permitted myself to hate was Superman, because he spoiled so many of my fine plans, and cost me many fortunes... All I want from my enemies is what they have in their pockets."
Sidney is willing to act as go-between, tracking down his old business associate the Scarlet Widow to obtain more kryptonite (which Miller had exhausted while killing Der Teufel). Sidney is dryly bemused and superior to Miller's short temper, expressing interest only in money. His only trusted confederate is his Japanese man-servant Jito, who also doubles as bodyguard.
Like Der Teufel before him, however, Sidney falls victim to the Atom Man's temper and power and is burnt to death by an atomic blast.