Milly Finch was JW's model.
JW painted three portraits of Milly, whom he called 'the red girl', in the mid 1880s (Harmony in Coral and Blue: Miss Finch (YMSM 237), Harmony in Fawn Colour and Purple: Portrait of Miss Milly Finch (YMSM 238), Harmony in Blue and Violet: Miss Finch (YMSM 239)). She also appeared in r.: A Note in Green; v.: Maud in bed (M.905), Harmony in violet and amber (M.906), Milly Finch (M.907), Arrangement in black and gold (M.932) and Note in pink and purple (M.935). She seems to have modelled for JW when Maud Franklin was ill.
She could have been an AMelia or a Millicent Finch. A Millicent Finch, aged 32, born in Islington, living in Edmonton, was listed in the 1901 census. However, this person was probably too young for JW's model, as she would have been only fifteen in 1884.
UK Census 1901; Young, Andrew McLaren, Margaret F. MacDonald, Robin Spencer and Hamish Miles, The Paintings of James McNeill Whistler, New Haven and London, 1980; MacDonald, Margaret F., James McNeill Whistler. Drawings, Pastels and Watercolours. A Catalogue Raisonné, New Haven and London, 1995.