Just dropping by in this discussion! I'm actually quite surprised that this drama seems to have many potential romantic interest given its genre, but romantic wise, I'm going with Kim Han Mi. They actually have a far more "natural" bond, comfortable with one another and were shown in the drama to have constant communication and scenes. Kim Kyu Ri seems more like a good friend to me and Kim Hee Ah (I'm still on ep 6), but I find her interest in Kim Hee Woo unnatural? It feels like to me that, she's the lead so she has feelings (probably crush) for Hee Woo? I don't know.
All of them of course looks beautiful and have visual chemistry with Hee Woo, but dynamics wise, I'm going for Han Mi.