She was a demon who fell for John Winchester. She fell hard, and she became entranced. She wanted him, she cared for him. So she decided to help him by breaking him out of hell so he could start over. But she had to go back and hide all traces of him–and that included wiping his memory.
She dropped by to visit this charismatic man every so often, admiring how he'd gotten along and started over. And especially how he'd managed to stay out of trouble. One day John (now Negen) approached her and asked her out to drink. She accepted without a second thought, and later drinks turns into kissing, and kissing turned into "Let's go to my place."
The problem was, the more he saw her, the more he started to remember his past. He couldn't remember who he really was. She was skilled, and her work wouldn't fall apart that easily. But there were traces.
His mouth was hot, his fingers fire inside her. It wasn't supposed to happened, but she accidentally called him John while he had her turning into putty against the wall. He had to push away. He paced the room, his back turned towards her. She feared he thought she was calling someone else. She feared she lost him, when she had done all of this, all of it, for him. When he finally faced her again though, rather than a face full of anger at being called by someone else's name, she found that he was fighting to keep tears at bay as he quietly asked, "Mary?"
He was so hopeful, so overjoyed. But all she could do was stare him down blankly. "No, she's gone…"
That was the night John Winchester died completely. That was the night the broken, sociopathic Negen started to take his place. By the time the zombies take over, she was the one to thank for the order and joy Negen takes in his methods. They always said everyone has a little devil on their shoulder telling them to do things they know are wrong. She just made it easier for him to not even consider the good. If only he had remembered his sons…