Rhaenyra Targaryen, the titular princess of the story.
The Princess and the Queen will be published in the Dangerous Women anthology, due out in December this year. At 33,000 words it's the longest story in the collection and clocks in at rather more than a tenth of the length of A Game of Thrones itself. It's a condensed version of a much longer story that Martin plans to publish in a collection - working title 'The GRRMarillion' - once A Song of Ice and Fire is fully completed. The World of Ice and Fire (due next year) will draw on both in its own account of the Dance of Dragons.
Martin is currently working on The Winds of Winter, the sixth and (hopefully) penultimate volume of A Song of Ice and Fire itself. In recent interviews he has said he is writing quickly, but not fast enough to give any indication of a release date.