Andrea is based loosely on a combination of Judson Caspian (the Reaper) and his daughter Rachel, characters from the Batman: Year Two comic book mini-series. The Phantasm's costume was designed by Bruce Timm as per Alan Burnett's initial description of a Grim Reaper-esque character. Burnett wanted the villain to be reminiscent of the Ghost of Christmas Future, a similarity that the Joker mentions in the film. Timm went through approximately 20 different character designs for the Phantasm before creating a version that would evolve into the final design.
Described by Timm as resembling a "tall, gaunt Death character", the Phantasm's costume is hooded and features a metal, skull-like mask. The Phantasm carries a large blade, reminiscent of a scythe, in her right hand. The blade was not part of Timm's original design, and was added later at Burnett's insistence. The character's left glove houses a chemically-based, artificial fog-generating device that the Phantasm uses for stealth, combat, and intimidation. At various points in Mask of the Phantasm, the Phantasm's manipulation of fog makes it appear that the character has paranormal powers, such as intangibility and the ability to disappear. These smoke-filled appearances were influenced by the Marvel Comics supervillain Mysterio. The Phantasm's facade is completed by a masculine voice provided by Stacy Keach, which was digitally altered in post-production to be deeper and echo.
The screenwriters of Batman: Mask of the Phantasm included Alan Burnett, Martin Pasko, Paul Dini, and Michael Reaves. Pasko wrote many of the flashback sequences (including those featuring Andrea) and Reaves the final showdown between Batman, the Phantasm, and the Joker.
In Mask of the Phantasm, producer Alan Burnett "wanted to do a big love story with Bruce because we hadn't really done it on the TV show. I wanted a story that got into his head." He also felt that the film's narrative should be removed from Batman's traditional rogues gallery. The resulting narrative, which dealt with Bruce's decision to become and remain Batman, hinged on the love story between Bruce and Andrea. Andrea's role in Mask of the Phantasm corresponds with that of the femme fatale from hardboiled detective fiction. Like classic femme fatales such as Brigid O'Shaughnessy from The Maltese Falcon, Andrea is a beautiful, intelligent woman who harbors dark secrets. The fact that Andrea is secretly the main villain is another convention of this stock character.
Andrea also serves as a foil for Batman. The characters are very similar; they both don an intimidating costume and speak to the gravestones of their parents (as if holding a conversation with the deceased). The paralleling of their characters — most importantly the death of their parents at the hand of criminals — highlights how different they are: Though they both became vigilantes, Andrea chose to become a killer, following a warped code of "eye for an eye" justice, while Bruce chose to fight crime without taking anyone's life. It's worth noting that of her four targets, Andrea only successfully killed one person in the film; Buzz Bronski, whom she crushed with a statue. Her first target, Chuckie Sol, died driving off the side of a parking garage. Valestra was already killed by the Joker before she arrived and Joker survived.
Andrea Beaumont is the daughter of wealthy businessman Carl Beaumont and his deceased wife, Victoria. In her youth, while attending Gotham University, she meets Bruce Wayne, and they fall in love. He is internally conflicted by his desire to lead a normal life with Andrea and keep the promise he made to his murdered parents to fight crime. He finally decides to abandon crime-fighting — instead pledging part of his inheritance to the Gotham City Police Department — so he can marry Andrea. As Bruce proposes to her at his estate, the two discover the cave that would later become the Batcave.
Andrea is forced to break off the engagement and move to Europe, however, when she learns that her father has double-crossed the Mafia. Carl had set up dummy corporations for some of the most powerful mob bosses in Gotham, including Chuckie Sol, Sal Valestra, and Buzz Bronski. When they learn that Beaumont has embezzled from them, they demand immediate repayment. Unable to quickly access the money he had stolen and subsequently invested, Carl flees with Andrea and eventually settles on the Mediterranean coast. By then, he has parlayed the money he had stolen into a sizable fortune. A few years later, however, when Beaumont's father refuses to make donations to Arthur Reeves, a politician formerly in his employ, the young councilman sells information on the Beaumonts' whereabouts to the mob. Although Carl has repaid them, the vengeful gangsters want "interest compounded in blood" and plan to use the information to have him killed. They send a hitman, who would later become the Joker, to kill Carl. Andrea encounters him as he leaves her villa, and discovers her murdered father only seconds afterward.
Consumed with hatred, Andrea waits patiently for many years, planning to destroy the men who ruined her life. When she returns to Gotham, she dons a costume reminiscent of the Grim Reaper, complete with a smoke-generating device and scythe-bladed gauntlet. Disguised as such, and mistaken by everyone as Batman, she kills Sol and Bronski. After Bronski's death, she revisits her mother's grave, only to encounter Batman, who is investigating the crime-scene where the Phantasm killed Bronski. Shocked to see her after all these years, Batman quickly departs, but Andrea notices that he was standing next to the grave of Bruce Wayne's parents and realizes his true identity.
Batman later discovers the link between the mobsters and Carl Beaumont and questions Andrea, but she rebuffs him. When the Phantasm attempts to kill Valestra, she nearly falls into a trap laid by the Joker, which she escapes. Batman attempts to apprehend her as she flees from Valestra's residence, although she easily evades him and the police. When Andrea realizes that the police will arrest Batman in her stead, she quickly changes out of her costume and helps him elude the authorities. In a final attempt to conceal her identity, Andrea tells Bruce that the Phantasm is her father and that she has returned to stop his murderous rampage.
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: "They took everything, Bruce. My dad, my life, you. I'm not saying it's right, or even sane, but it's all I've got left. [...] They had to pay!" 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 bids farewell to Batman, and disappears with the Joker.
After escaping the inferno through a series of sewers, Batman assumes that Andrea and the Joker have perished in the blaze. However, she survives and leaves a locket for Bruce in the Batcave as a keepsake. In the penultimate scene of the film, Andrea stands alone on the deck of an ocean liner at dusk.
In conjunction with the release of Mask of the Phantasm in 1993, Kenner released a Phantasm action figure. However, it was packaged with Phantasm's mask and cloak off, revealing the character's identity as Andrea Beaumont, spoiling the film's mystery of the villain's alter ego.
The Phantasm makes her first returning appearance in the comic of Batman & Robin Adventures Annual #1 (1996). The Phantasm also appears in Batman Adventures: Shadows and Masks, in which Andrea goes undercover in the 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. She eventually fights Batgirl, who takes some of her gas pellets. She also tries to kill Black Mask. She later tricks Arthur Reeves into killing himself.
The Phantasm returns in Mark of the Phantasm, a seven-part storyline of the digital comic Batman Beyond 2.0 set after the Batman Beyond animated series. In the first issue of the arc, the Phantasm seeks out the great-grandnephew of Joe Chill, Jake Chill a.k.a. Vigilante, for the murder of Terry McGinnis' father Warren.In Batman: Li'l Gotham, Phantasm's mask was placed on display in the Batcave.
In DC Comics Bombshells, an alternate history version of World War II, Andrea Gruenwald, daughter of Dr. Benjamin Gruenwald, assumed the identity of the Reaper to take revenge on Hugo Strange, who was responsible for the death of her parents. Her quest against Strange leads her to confront the Batgirls, as well as, Penguin and Killer Frost.
The Phantasm reappears as an assassin in the Season 4 finale episode of Justice League Unlimited ("Epilogue"). Now an elderly woman, the Phantasm is hired by Amanda Waller to murder Terry McGinnis' parents in front of his eyes. The psychological trauma, Waller reasons, would have steered Terry toward becoming Batman's successor. However, the Phantasm cannot go through with the job and abandons it moments before she would have slain the young couple, unable to tear apart a child's life the way Bruce's life (as well as her own life) was. The Phantasm argues that the murder would defile the Batman legacy by breaking Bruce's paramount rule, to never take a life. Her sparing of the McGinnises would indirectly result in the birth of Matt McGinnis, Terry's brother and Bruce's second genetic offspring with the Cadmus Project. Her refusal marks the official end of Waller's "Batman Beyond project", only to have fate compel Terry to eventually become Bruce's successor anyway, thanks to the misdeeds of Derek Powers. In the episode, Andrea has no lines or voice actor; her sentiments are conveyed through Waller's monologue recounting the event. Her scythe-gauntlet in the future has glowing orange studs on it, suggesting some degree of technological overhaul.
Andrea Beaumont is mentioned as an Easter egg in Batman: Arkham Origins, where postcards from Rome and Paris addressed to Bruce Wayne bear her signature.