Hemingway does not give details about the appearance of either character. The most we are told is that it is an American and a girl, and that the girl was wearing a hat - a detail that says more about the time period than it does about the girl herself.
The reason that Hemingway does not give details is very specific. Hemingway was a modernist writer who believed in simplicity. Unlike the authors who came before him, Hemingway's style used a lack of detail and a lack of wording in order to get his point across. This is known as imagist writing - the simple images and words provided are meant to convey emotion. The emotion in this story is that of conflict, and of ambiguity. The girl and man are at a crossroads and don't know how to communicate about it to one another, much less how to proceed. That uncertainty is echoed in our ignorance of their past and their appearance - even their names.