When he first married Hee Joo he told her she was just his hostage. Then when 406 called him at the ambassador's dinner, he threatens the kidnapper telling him how dare he hurt "my person". So basically she was his hostage, then became his person.
BSE: "You could get hurt and be in danger again if you stay with me, but I' m asking you to stay by my side because I can' t let you go, I lived because of you. I endured because of you"
BSE: "You could get hurt and be in danger again if you stay with me, but I' m asking you to stay by my side because I can' t let you go, I lived because of you. I endured because of you"
BSE: Why was I born? If I have to live someone else’s life against my will, why was I born? But I think I found the answer to why I was born and why I had to live as Baek Sa-eon... It was… to meet you.