ETERNAL LOVE (2017)
"Even if you have to pay with your life for the bad things you have done, I will follow you in death. Here, in this world, no matter when, no matter where, as long as you do not betray me, I will never leave you."
And that is the crux of why he's obsessed with her - it's not just the brains or the battle prowess or being able to match him in an emergency or saving his life or her attractiveness or w/e.
It's because she's showing him you can keep yourself and your morals and still forge ahead - and it's not from a stupid person or a naive one or a weakling; it's from the one person who can match him fight for fight and plan for plan. And yet she consistently is certain in herself and in her soul and that is catnip to him.