Beldam is the leader of the three Shadow Sirens which consist of herself and her sisters Marilyn and Vivian (who was later replaced with Doopliss). Beldam is a demon who is more than 1000 years old and adept at magic; she also has a pretty bad personality. She constantly mouths off about how pretty she is and she is always telling Vivian she's ugly and useless. Beldam and the Shadow Sirens worked with Grodus to find the Crystal Stars and open the Thousand-Year Door pretending to be nothing more than hired help. However, Beldam secretly planned to betray Grodus and defect to the Shadow Queen, the ancient demon sealed behind the Thousand-Year Door.
Beldam was the one who kidnapped Princess Peach and the one who spread the rumors that there was a great treasure behind the Thousand-Year Door. Beldam and the Shadow Sirens appeared throughout Mario's adventure trying to slow Mario down. Beldam eventually revealed her plans to aid the Shadow Queen in conquering the world when Grodus unleashed the demon. Beldam took all the credit for releasing her. When Mario defeated the Shadow Queen, Beldam suddenly became nice and now lives in Twilight Town with her sisters.