"Fanny" Watts (1858-1927) was a childhood friend of John's. Her parent's, as were John's, were all part of a loosely associated group of expatriate Americans living essentially permanently in Europe, traveling from one city to another with the seasons and on a whim. They would hit and miss each other at the various locals throughout the years.  Where they found themselves together, they tended to socially gravitate together in an undeclared extended family and would loosely keep in touch with each other if they were separated to different places.

Fanny was in this smaller group of children of these expatriates along with John, his sister Emily, Violet Paget ("Vernon Lee") and Ben del Castillo.

The Watts found themselves in Paris the winter of 1877 and it became rather natural that John would use her for this portrait. The painting was done under the tutelage of his teacher and probably, at least in part, at Carolus-Duran's atelier. 
 

Notes:

Exhibitions

John Singer Sargent, An Exhibition -- Whitney Museum, NY & The Art Institute of Chicago 1986-1987

Sargent's Women,  Adelson Galleries, New York, 2003

• See the Philadelphia Museum of Art
• See the year in review 1877