My interpretation of Plague and Cure in their first few appearances has always given off the impression that care is there on his side, at least a little bit. Yes it's mostly derived out of possession in the beginning, but even if he won't admit it to himself having Cure there has been a big change in his life that he doesn't want to lose. I saw him allowing her to ask about his neck scar as him unlocking a door so to speak, and starting to acknowledge she's more to him than something to be owned. This last chapter with Ren shows that even more clearly. Plague already admitted he knew that making Cure his second wasn't anything that would stop his promotion, he just trusted her and even wanted her to get something out of it. He could've just used her unofficially if he didn't and she wouldn't be getting the pay and status other DH seconds get. If he really knew that would open her up to harm for any more mistakes I bet he never would've made it official.
Cure may simply see it as an obligation, her feelings are harder to parse but she's not as much in a position to feel anything less than complicated about him. This is just her life now.
I imagine Plague as someone who is very lonely and not emotionally matured. Which then comes across as aggression and being a little bitch. Which then leads people not liking him. Which then leads to him taking Cure as a payment because he is entitled like that in his own mind and because that's a way to secure at least some company.
If he was a really bad guy who wants to own a person because it gives him limitless freedom to do anything he wants with the said person, he would have done horrible things to Cure. No one's going to stop him, he legally owns Cure. He could go a full psychopath mode with her without anyone stepping in.
But he doesn't.
Of course, he is not obviously aware of his behavior and where it comes from.
But that's only my current understanding of Plague.
When it comes to Cure, for me she appears as a defeated person who has accepted her faith, and is likely embarrassed to be just an object to be owned. She has not told her parents her true situation, only that she got a new job (when Plague claimed her as his payment).
