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. So either help me or get out of the way!
~ Andrea explaining her motives to Batman.
Andrea Beaumont, also known as the Phantasm, is the titular main antagonist-turned-anti-hero/supporting character in Batman: Mask of the Phantasm. She is a former lover of Bruce Wayne turned deadly vigilante, who targets the mob bosses responsible for her father's death. She also fights Batman for getting in the way.
The Phantasm was voiced by Stacy Keach, who would later voice Robert Vance in Batman Beyond, as well as also physically playing Cameron Alexander in American History X and Ken Titus in Titus. Andrea was voiced by Dana Delaney.
Bruce Wayne was visiting his parents' graves at a Gotham cemetery when he met the beautiful Andrea Beaumont. They quickly fell in love and began spending time together.
At the same time, Bruce was also beginning to fight crime as a masked vigilante, but no one was scared. He decided that bats were scarier and attempted to be 'Batman'. After stopping a street robbery, Bruce gets hurt and Andrea is frightened.
Bruce returns to his mansion, angered, as he replied "I can only have one or the other, I can't have it both. I can't go out doing this if I have someone waiting for me to come home every night!" Deciding to choose love over crime fighting, Bruce proposed to Andrea.
Although initially accepting Bruce's proposal, the next morning, Andrea returns the ring before leaving Bruce and Gotham behind out of the desire to not let anything keep her from avenging her father's death. Saddened, Bruce ultimately donned the mask of Batman.
Because of the dark appearance, the Phantasm is mistaken for Batman and the Caped Crusader is put on the run from the police, attempting to clear his name. Valestra went to the Joker and pays him millions of dollars to kill Batman after Buzz Bronski's murder, although Joker would end up killing him instead. The Joker learns that Andrea is the Phantasm.
Andrea is killing the gangsters for the following reason: Andrea wanted to marry Bruce, but Salvatore and his gang was pressuring Andrea's father to give them the money he owed them.
He knew he would be unable to do so, so he and Andrea moved away to the Mediterranean Coast of Europe to hide. His father eventually paid them. However, it apparently was never enough and when Andrea returned home after buying groceries one day, she found her father had been murdered (by the unnamed hitman who would later become the Joker).
She became the Phantasm and killed Valestra's gang as revenge. Bruce (as Batman; Andrea knows his identity) confronts Andrea and asks him "what will vengeance solve?" Andrea responds "if anyone knows the answer to that Bruce, it's you!" A battle then ensues between Batman, Joker and Phantasm. Joker vanishes into the night, laughing as it happens, and Batman escapes an explosion, using a waterway below.
Batman thinks that Andrea died in the explosion, however, in reality, she escaped to a cruise ship where one of the passengers notices her standing alone at the rail. He starts to introduce himself, but notices her rather cold greeting, and mourning veil. When he apologizes and asks if Andrea wishes to be left alone, she answers that she already is. It is unknown why she spared the Joker. However, she possibly did it because of his mental illness.
Sometime afterwards in years that have passed, Andrea was hired by Amanda Waller to murder Terry McGinnis' parents so that he would follow in Batman's footsteps. However, Andrea could never bring herself to do it, as it was everything that Batman stood against and possibly because she will never go as far as to kill someone's parents.
In many ways, the Phantasm is considered the dark reflection of Batman: an example of what Bruce Wayne would have become had he lost all those who loved him and chose vengeance to fill his grief.
The producers of the movie were so impressed with Dana Delany's performance as Andrea Beaumont and enjoyed working with her so much that they cast her as Lois Lane for Superman: The Animated Series. Ironically, Lane also goes to date Bruce Wayne, but unlike Andrea, they parted on good terms.
Before the release of the film, a toy line was unveiled, which included a spoiler as to the true identity of the Phantasm: The package had included Andrea Beaumont and with the Phantasm costume right next to him.
Phantasm also made a few appearances in the DCAU comics. One of which reveals how she and the Joker survived the explosion in the climax of the movie.
Whenever Bruce reminisce about his past loves, Andrea was left out. Presumably because he only sees her as a "killer" in one of the DCAU comics.