The Sister We Fought Previously
In addition to our protagonists, there were others who explored the Cathedral and attempted to purify the Outer Ones, including the person who left behind the formula for the Great Purification for Hanya to analyze. However, they all fell victim and became background objects or enemies in the stage.
The Sister who is the boss of the tutorial stage is one of them, and at the very least, she's someone who's been close to the final boss.
Whether she's the same as the person in the Great Purification is unclear, but what I wanted to achieve here is a bad ending route in which losing to the final boss would result in the protagonist becoming the same as the tutorial boss.
We don't know why she was in the convent, but perhaps she was trying to somehow purify herself, or perhaps she was trying to seal herself away before she became a complete monster.
Well, it's easy to say that this is just for gameplay purposes, but personally, I like to imagine these kinds of things.