For everyone who's saying their relationship doesn't work...the main issue is making it more clear the switch in Cecilia's perspective on Youki.
Because there was one at some point, it is clear in the way they interact since like 10 chapters ago and how they interacted on the initial 10 chapters, and it was clearly her leading him on to prevent him from returning to being a demon.. up until a point where she started to take him more seriously as a person instead of as a threat to safety and peace.
But it is extremely hard to pinpoint that moment, because of how easily she seemingly accepts everyone else but him, and how she doesn't really stop blushing or feeling awkward about interacting with him. It's just that at some point, it stops being because he feels like someone she doesn't know being overly friendly with her for uncertain reasons, and starts being because she is genuinely charmed by the person he is while becoming less and less guarded against his sincerity.
She actively starts believing in the person he has shown himself to be the entire time, the core of the person he wants to be, instead of thinking it is a ruse or something that can go south at any time. But we weren't given any noticeable sign of when that happens. Which IS BAD. Because there is clear reasoning BEHIND the how, but it doesn't feel like the HOW itself ever happened.
At most, we have to believe that that one time were they didn't meet each other for a while was enough to give her perspective on Youki's actions and his way of being, and helped it happen...emphasis on "believe", because the author of the manga missed the point where we need to see Cecilia start thinking about him and properly start to sort out her feelings for him towards something more romantic to make this development believable.