Mary Campbell was a hunter who existed in the Apocalypse World.
She was the alternate reality version of Mary Winchester.
In 1973, the Prince of Hell Azazel killed Mary's boyfriend John Winchester and offered Mary a deal to resurrect John in exchange for permission to enter her house in ten years. Unlike in the Main Universe, Mary rejected the deal and John remained dead. Mary never moved on lived the rest of her life alone and regretted not taking the deal that Azazel had offered her.
When the Apocalypse happened, as a result of Mary never making her deal, Sam and Dean were not around to stop it. Mary fought alongside fellow hunter Bobby Singer and saved his life "more than a few times." According to Bobby, when Mary got drunk enough, she would tell the tale of her loss of John. She was eventually killed by Azazel in about 2007.
In 2017, Bobby encountered Sam and Dean and recalled some of his knowledge of Mary. Six months later, after Jack and Mary Winchester escaped from Michael's fortress, they encountered Bobby who mistook Mary for her Apocalypse World counterpart. While discussing her alternate counterpart with Bobby, Mary learned of her alternate self's failure to make the deal and what occurred as a result. After being told that Sam and Dean were able to stop the Apocalypse in their world, Bobby reassured Mary that she made the right choice, something that comforted her after seeing the results of her alternate self's decision.
Bobby Singer described this version of Mary as "a complicated woman. Brave, but sad. Full of regret." Like Mary Winchester, her version of John Winchester was the love of her life and she never moved on after he died.