Isabella

The note slip Lucien saw in the bathroom is the same note Lisa give to Isabella. On her wedding day, she secretly went up to Isabella's room who was transferred from the attic to the basement and freed her. But the priest saw what Lisa did so she hurriedly ran towards the bathroom and lock the door to prevent the fake priest from harming her. 

Lisa knew she could not live long. So rather than being married by a demon, she would rather risk death by doing something right for once. After all, due to the fear of Gabriel and the priest, she turned a blind eye to Isabella's torture in their hands.

Lisa is selfish. But like human beings, she wanted to survive. She doesn't want her heart to be taken out and used by the priest to replace Isabella's contaminated heart. 

The fake priest followed Lisa to the bathroom and pounded on the door forcefully. For a few minutes, Lisa listened to him trying to break the bathroom door open. After a while, he suddenly stopped.