Warrior Nun showrunner Simon Barry has opened up about original plans for Shotgun Mary, which would have seen her survive into season 2 while taking another lead character's role. In season 1, Mary is introduced as an ally of the OCS, someone who fights for the same cause as the group without being a nun. She died before the events of season 2, succumbing to an army of Adriel's Wraith Demons.
https://screenrant.com/warrior-nun-show-shotgun-mary-death-survive-showrunner/