Even if Athit isn't serious with Aerin (forgot her character's name), he should still be considerate of Wilai. A girl never wants to be a guy's option. And by Athit carelessly flirting with other girls and giving them the wrong impressions, it leads Wilai to believe that she's not the only girl he wants. On the other hand, I think Wilai is really interested in Athit. I don't know what she sees in him (the directors aren't making it clear enough), but perhaps it's his personality. From the start Wilai really didn't want to lie to him about who she was. I like that she wants to be truthful with him. It shows that she cares enough to not want to deceive him into thinking she's someone she's not.
I think Wilai sees Athit as her prince charming. He's supposedly rich, of high status and he's good looking in her eyes. And who can blame her? It's a bit shallow, but her story is kind of like that of Cinderella's. She comes from a poor family, has lived as a servant all her life and out of nowhere, a man with all the perfect qualities appears in her life and shows an interest in her. She feels like she's living the dream.