Prior to the arrival of Batman and Rupert Thorne, Salvatore Valestra ruled Gotham with an iron fist. He had Chuckie Sol and Buzz Bronski as his enforcers. Salvatore loaned money to financial planner Carl Beaumont. Despite promising he could, Carl was unable to pay Valestra back. Not long after Carl's daughter Andrea became engaged to her sweetheart Bruce Wayne, Carl had both of them flee the country to Europe in order to escape Valestra's wrath. Valestra vowed to "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 in exchange for Salvatore give Reeves' campaign a financial boost and giving him inside information on the Gotham city council, making Reeves have some control over the Gotham political machine and Salvatore with a friend in high places.
Years later, Valestra lost his grip over Gotham's criminal underworld to new criminals such as Rupert Thorne, Penguin, Two-Face, and Count Vertigo. Additionally, his former enforcer Jack Napier now become the Joker. Batman and Commissioner Gordon were also both causing problems for Salvatore as they were beginning to crackdown on all the mob bosses of Gotham City. Due to his smoking, Salvatore began suffering from respiratory problems.
After Chuckie and Buzz were murdered, the public blaming Batman led Valestra to blaming him as well. Valestra was unaware of this, but the culprit was actually Andrea Beaumont, who was doing it under the alias of "the Phantasm" in revenge for Valestra, Chuckie, and Buzz killing Carl, who was her father. Reeves sensed that Valestra was loosing control of the Gotham criminal underworld and refused to help him since Reeves felt he didn't need Valestra anymore. Deciding to take matters into his own hands, Salvatore tracked the Joker down to the abandoned Gotham World's Fair and went to him for help. 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 presumably incinerated in the explosion that followed afterwards, while the Phantasm evacuated.
Bruce eventually does learn that Andrea is the Phantasm after Reeves confirms that her father is dead after Andrea lied to Bruce that her father was the Phantasm. Andrea tracks the Joker to his hideout at the derelict Gotham World Fairgrounds, but the Clown Prince of Crime has deduced her identity, and is ready for her attack. The two fight hand-to-hand in the "Home of the Future" exhibit before he lures Andrea to a giant turbine. He attempts to kill her by sucking her into it, but Batman intervenes, breaking the machine moments before its blades would have crushed her. Andrea attempts to justify her actions. She asks him to let her carry out her vendetta without interference. For once, completely helpless, Batman begs her to leave. Andrea agrees and disappears in a cloud of smoke. After a battle between the Dark Knight and the Joker, Andrea reappears to claim the defeated maniac. As the fairgrounds explode, Andrea disappears with the Joker, with the latter laughing maniacally. Andrea and the Joker both survive.
In the DCAU comics, Andrea does return to Gotham to protect Bruce from a criminally insane Reeves, who was poisoned with Joker Venom and has put out a hit on Bruce to lure Andrea back to Gotham right after Arthur figures out her identity. Arthur tries to kill Andrea only for Andrea to trick Arthur into committing suicide by jumping out a window from an enormous skyscraper. Andrea would also later on suspect her mother Victoria to be alive and leading Black Mask's organization, the False Face Society. During this story arc, she confronts Bruce at Wayne Manor and asks him not to interfere with her operation. He rebuffs Andrea, referring to her as a "killer" and thus unworthy of his consideration. Andrea tries to kill Batgirl and Black Mask only to be failed by Bruce.
Valestra's death was finally avenged when the Joker was murdered by Tim Drake, who became Batman's new partner. Joker and Harley Quinn were obsessed with having children together, so they captured Tim to make her his new son. At the old Arkham Asylum, Tim was mercilessly tortured with electro-shock and chemical injections. The Joker also took the opportunity to implant a microchip at the base of Tim's skull that contained an encoded version of the Joker's DNA and memories. During the course of the torture, the Joker learned the precious secrets given to Tim. Not content with just destroying Tim's mind he sought to further humiliate Batman. To this end, the Joker transformed Tim into a crude version of himself. Tim's skin was bleached white, dying his hair green and twisting Tim's mouth into a grotesque Joker-like grin. Because of the torture and makeover he endured for the three weeks, he was also rendered unable to communicate in any way except with demented laughter. Renaming him "Joker Junior" aka "J.J." the Joker claimed Drake as his and Harley's new son, and planned on using Drake to destroy Batman. During the battle that followed, the Joker gave Tim a gun and ordered him to kill Batman with it. Tim laughed uncontrollably and fought with the choice whether or not kill Batman. He finally killed the Joker instead, after which he completely broke down in the arms of Batgirl. After these events, Commissioner Gordon quickly covered up the incident and nothing was ever made public.
Not long after Bruce's retirement as Batman, Andrea eventually starts working for Amanda Waller and the United States Government. Waller became obsessed with making Terry McGinnis be the new Batman and ordered Andrea to kill Terry's parents in front of his eyes. Andrea refuses to harm Terry's family and persuades Waller to cancel the project by pointing out that Batman would never resort to murder.
Salvatore Valestra is very similar to Rupert Thorne as they are both enjoy their height of power and control over others. Valestra is arrogant to the point of being overconfident as he believes himself to be inferior to everyone as he rudely barged in Carl Beaumont's office without an appointment while intimidating Carl's secretary. Valestra even rudely smoked in Beaumont's office and blew smoke right at a young Bruce Wayne's face. Despite Valestra being remorseful that Andrea found out about Carl's crooked dealings, Valestra still opted to kill her in order to punish Carl unless Carl paid up. Despite Carl eventually paying Valestra, Sol, and Bronski back, they were still angry Carl for stealing from them and wanted revenge. Valestra still wanted interest compounded in Carl's blood which is why Valestra sent his chauffeur to kill Carl in Europe.
Despite being enraged at Carl's disloyalty, Valestra was still treacherous towards Carl even when Carl was still loyal to him. In the unfinished DCAU storyline, it was revealed that Valestra, Sol, and Bronski took orders from Carl's wife Victoria Beaumont, who was their boss. Victoria faked her own death in order to protect her husband and daughter from her life. When Carl became involved with the Valestra mob, Valestra never told Carl about his wife's involvement with the organization.
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.
It is possible that (similar to Roland Daggett who was based on Max Shreck) Sal Valestra was inspired by the Burtonverse character of Carl Grissom from the 1989 film. Both characters are aging crime-lords that were Gotham's kingpin in their prime and employers of Jack Napier before he became the Joker. Additionally, both Valestra and Grissom are murdered by the criminal, after his tranformation.
Valestra's first name being Sal is possibly an allusion to the character of Sal Maroni, a prominent Gotham gangster that was responsible for the creation of Two-Face. In Batman: The Animated Series, this role was given to Rupert Thorne.
Valestra is briefly mentioned in the Elseworld story Batman: White Knight, where his gang is one of many taken over by Jack Napier in his political-scheme to make Gotham City a better place.