Prior to the arrival of Batman, Sal ruled Gotham with an iron fist. He had Chuckie Sol and Buzz Bronski as his enforcers. A man named Carl Beaumont didn't pay them back for loaning them and Sal made threats. Carl promises to give the money in 24 hours and Sal agrees, on the condition that if it happens otherwise, he would "have Carl's heart in his hand". Carl took Andrea and left to the Mediterranean and managed to send the money. Sal later realized that Carl embezzled a fortune and sent a hitman to kill him. The hit was successful due to Arthur Reeves who gave them the information.
Years later, a vigilante named Batman came into Gotham and pressured Sal's business along with Comissioner Gordon. Not only that, but Valestra's enforcers were later killed off one by one by a new vigilante named the Phantasm.
Fearing for his life, Salvatore went to the last person who could help him, the notorious criminal and his own former hitman, the Joker, though even he was afraid of his former henchman. Valestra offered the Joker five million dollars to kill Batman but the Joker was not interested. Sal made a big mistake by grabbing the Joker's shirt and getting angry at him. The Joker gleefully assured Sal that he would take care of all his problems and that no would hurt his old pal Sal. The Joker later killed Sal by using his Joker venom and used the mob lord's body as bait. The Joker found out in a camera that the killer was not Batman but Andrea Beaumont using the identity as the Phantasm to kill the mob for killing her father. Sal's body was then incinerated in the explosion that followed afterwards, while the Phantasm evacuated.
Salvatore Valestra was mentioned in The New Batman Adventures.
Valestra was nicknamed "the Wheezer" for his lung condition, presumably the results of his chain-smoking when he was younger.
As the Joker's former boss, Valestra may have been the last person who knew the Joker's real name and identity. This may be an added reason for the Joker's decision to kill him.
Valestra was likely named after Salvatore Tessio from The Godfather, another mobster who was also played by Abe Vigoda, Valestra's voice actor.
Valestra is the overarching antagonist of Batman: Mask of the Phantasm because if he hadn't sent the Joker to kill Carl Beaumont, Andrea wouldn't have become the Phantasm.