Perhaps it is the famous luck of the Irish that enabled Mad Sweeney to become the breakout character of American Gods season 2? Certainly, the leprechaun (played by Pablo Schreiber) is a likable fellow and the only thing that is surprising about his popularity is how minor a player he was in the novel that inspired the Starz! series.
Originally published in 2001, Neil Gaiman's American Gods told the tale of Shadow Moon - a convict whose plans for a new life upon release fall apart after his wife and best friend are killed in a car crash. Shadow falls into the employ of Mr. Wednesday, who is eventually revealed to be a modernized aspect of the Norse god Odin. It is Wednesday who draws Shadow into a world of magic he never knew existed and the brewing war between the new gods of America and the old gods and creatures of classical mythology, including Odin and Mad Sweeney.
In the original novel, Mad Sweeney appeared in only two scenes. Early on, he challenges Shadow to a fight at Mr. Wednesday's request to see if he is tough enough to act as Wednesday's bodyguard and gives Shadow a gold coin after trying (and failing) to teach him the true magic trick of pulling gold from out of thin air. Later in the novel, Shadow encounters a near-dead Mad Sweeney, who explains that he accidentally gave Shadow a magic coin that can only be owned by royalty and he's been cursed with ill fortune by giving it to Shadow. He begs Shadow to return the coin to him, but Shadow no longer has it, having tossed it into the grave of his late wife, Laura, at her funeral. Mad Sweeney dies shortly thereafter, freezing to death on the street after spending all the money Shadow gave him for "a ticket out of here" on a bottle of whiskey.
Mad Sweeney's story was changed significantly for American Gods season 1. The fight between Shadow and Mad Sweeney plays out as in the novel, but Mad Sweeney realizes his mistake far sooner and is able to catch up with Shadow and learn where the coin went. This leads him to encounter Laura Moon, who was resurrected by the magic of the coin after Shadow gave it to her. Since the rules of magic will apparently make the coin stop working if the coin is stolen rather than being given freely, Mad Sweeney enters into an odd partnership with Laura, promising to help find a way to truly resurrect her in exchange for her giving him back the lucky coin.
The relationship between Laura and Mad Sweeney, whom it should be noted once again never met in the original novel, was one of the high points of American Gods season 1 and continued into the first part of season 2. Yet it was "Muninn," episode 3 of American Gods season 2, that proved Mad Sweeney's staying power as a solo act. With Laura Moon deciding to go with Mr. Wednesday instead of following Mad Sweeney's plan to seek out a friend of his in New Orleans whom he says is very good at raising the dead, the leprechaun elects to go it alone and make his own way southward. Hilarity ensued as Mad Sweeney's cursed luck thwarted his every effort, from trying to steal an unlocked car only to find a vicious dog in the backseat, to nearly getting burned alive after the boat he was stealing caught on fire.
In this, Mad Sweeney invites comparison to Daryl Dixon from The Walking Dead - another character who slowly took over the series where they were meant to be a supporting player. The two differ in that Daryl was specifically created for television and never appeared in the original Walking Dead comic books, whereas Mad Sweeney had a role in the source material of American Gods, albeit a minor one. Despite this, the leprechaun has captured the hearts of fans everywhere, proving he may have lost his lucky charm but he still has charm enough.