I'm assuming he came to earth to mingle and she could see the "real" him somehow. . Maybe he used her knowing what he was to mess with time and space (and her mind)? This ends up scaring her a lot, causing her to curse him out and fight back in interesting ways. Her bluster and boldness made him like her. He starts liking her and instead of messing with her head, it's molesting her and showing "love". She fights it as best she can, but it's a lot. Finally she's had enough and she threatened him somehow which leads to him posing the whole you-get-amnesia-and-gain-a-perfect-life trope from dramas, but with the promise of no more space and time mind games, just games in bed. d. She agrees to stop the cosmic horror and he pulls out all the memories of her life and destroys the world before her eyes- creating the one we see. Cue the start of the story from there...?
kidding aside, do yall have a theory of what he's hiding?