The figure is Sargent's younger sister, Violet (later Mrs. Francis Ormond). The picture was almost certainly painted at the home of his parents in Nice. It appears on an easel beside Sargent's Madame Gautreau in a photograph of Sargent in his studio of 1884, in the Smithsonian Institution, Washington DC.
Other alternate previous titles:
"Miss Millet at Breakfast" (letter from Birnbaum) / "Miss Violet
Sargent, Nice, 1884" (registrar's records). 6/94: Ivan Gaskell notes
that the Jeune Fille title seems to be the most historic but it is not
traceable back to Sargent's time, and it has become most well known as
The Breakfast Table.