The Mistress of Mystery is the Silver Shroud's former lover and companion while the Shroud terrorizes the criminals of Boston. She can be heard talking to the Shroud and some criminals on Silver Shroud radio. Like the Silver Shroud, the Mistress of Mystery is a fictional character created by Hubris Comics. They also appear together in the Unstoppables.
She is shown to carry a .44 revolver called the Voice of Set on the Unstoppables covers and mentions using a weapon she calls the Blade of Bastet in the radio show. She's also an accomplished unarmed fighter, mostly relying on kicks. She wears long black gloves, a black evening gown with a white collar of several styles, and black heels. She tends to speak in alliteration. In Fallout 76, it is revealed that she used a Phantom Device for escapes.
Her radio voice actor was Shannon Rivers, who was also poised to portray the hero on the Silver Shroud television series. Producer Aaron Babowski replaced her with Claire Redelle, but the Great War prevented filming. Shannon and her husband Frederick survived the war and founded the Order of Mysteries based on the ideals of the fictional hero. The Order members were herself, her daughter, and many orphaned girls. They each wore and used recreations of the hero's equipment developed by Shannon's husband.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
There are noticeable similarities between this character's name and that of the DC Comics character Wonder Woman. Both names have similar meanings and the initials WW and MM are mirror images of one another.
End of information based on unverified behind the scenes information.