I focus on personality traits. In Eileen’s case I knew she’d be as awkward as they come, often to her own detriment, but she would also be brave, honourable and prepared to make considerable sacrifices for causes she considers worthy enough. So being prepared to give up what the Queen considered a privileged lifestyle for a much more humble life on Earth with a mere human male is something someone like Eileen would do without that many qualms.
I have since fleshed Eileen out physically with chestnut hair, a good figure and great skin. But I do think the physical attributes are less important (though Eileen has been known to use her physical wiles to help her cause, especially when she was living in the Kingdom). Had I been stuck for physical attributes, one good idea often repeated in writing circles is to take photos from magazines and so on and “blend” different features from pictures you like to create your heroine. I’ve found it helpful to “see” my heroine in action, doing things, speaking (and getting a good idea of her voice did help give me her class and as a result ideas as to what she’d look like – she’d be unlikely to be a blonde bimbo) and from that other aspects of Eileen’s life, including her looks, fall into place.
The way Eileen speaks tells me so much about her - she's well educated, highly bred (but also uses a sensible, easy style of speech so she's trying to show the common touch) and can argue her case well. She does, often! She, to the disgust of the Queen, will indulge in a swearing bout on those days when things have not gone so well. It's not the language that bothers the Queen, she's used stronger herself, but it's the fact that humans would behave like this and she doesn't relish the comparison. Eileen doesn't care. She doesn't care much about what people think and has found this has given her considerable freedom of speech and action. People expect her to be awkward so almost anything she does now will not come as a surprise and she knows how to take full advantage of that.