Sister Mary Catherine was a nun at the St. Bonaventure Convent in Milton, Illinois.
In 1958, the Knight of Hell Abaddon arrived at the convent and took possession of the Mother Superior. With the help of her demons, Abaddon began extracting souls from the nuns at the convent, Mary Catherine becoming one of the first victims.
In her soulless state, Mary Catherine drew all over the walls of her room in blood and carved the crest of the Knights of Hell into the wall as well. Mary Catherine eventually killed two other nuns and then committed suicide by jumping from the bell tower.
Mary Catherine's strange behavior drew the attention of the Men of Letters who suspected demonic possession. The Men of Letters sent initiates Henry Winchester and Josie Sands to investigate. On the wall of Mary Catherine's room, the two found the Knight of Hell crest, worrying Henry. Henry and Josie ultimately exorcised two of Abaddon's demons before Abaddon possessed Josie and Sister Agnes played dead, allowing Henry to believe that they had beaten the demons responsible for what had happened.
In 2014, Henry's grandson Sam Winchester investigated a similar series of events. Julia Wilkinson, a former nun who had helped Henry and Josie investigate Mary Catherine, informed Sam of the events and Mary Catherine's roll in them. Sam eventually learned from Sister Agnes, still operating Abaddon's soul factory, what happened to Mary Catherine and the other victims.