Mary Barrows gave birth to two twins, Bobby and Dan with the help of Dr. Walter Simpson in 1986. However, they were not normal twins, as they devoured Dr. Simpson's right hand. He immediately discovered that the children were demons, and they should have died. In result, Mary had Dr. Simpson sealed in a padded room that was intended for her, where he died three days later of suffocation. Sometime before the first game began, she locked Simon Barrows in the shed within the courtyard of the mansion.
Mary adopted four orphan girls, Jennifer, Ann, Laura, and Lotte from the Granite Orphanage to live with her and Mr. Barrows at their mansion in Norway. When they arrived, she had them remain in the main foyer while she left. It is assumed she was scheming the demises of the girls, along with Bobby, to feed them to Dan.
She can be seen in many ways in the first game. In the telephone room, she can either "comfort" Jennifer by giving her a drink, which poisons her and locks her in the shed, where she later kills Lotte and tries to kill Jennifer. If Jennifer found out about her father's demise, Mary will simply try to kill Jennifer in the telephone room. Mary met her demise upon the clock tower (depending on the path chosen, she can either be flung off by ravens, thrown into the circuit board and shocked to death, or flung off the clock tower in attempt to kill Jennifer.) In some of the endings, Mary either kills Jennifer herself, or lives. Canonically, Mary was killed in the incident, as reports say that Jennifer was the only survivor.