For me, the only unrealistic part of their relationship is the fact that Maou was 100% bluster when she seduced him, and that they are both shy still. They haven't even gone beyond touching hands in a year. She might be older than he is (by possibly even a century...), which makes it harder to believe that they are both that naive and shy about the love they are supposed to share for each other.

You can see why the head maid is getting frustrated with the two of them.

I feel it would be much stronger if the romance was more one-sided. One could argue that it already is with the Maou keeping body pillows and going crazy while Yuusha is content to write letters picture diary entries (but come on, you can admit that was funny) to her in her own office. But I mean more substantially.

Now, had she seduced him with her body all the way after a bottle of "juice" that first episode, then maybe I could understand the way this series is.