Mildred Howells was a painter, watercolourist and poet, and the sister of J.M. Howells.
She is referred to in JW's correspondence about 1895. She posed for a portrait, A Portrait: Mildred Howells (C.112).
Spink, Nesta R., The Lithographs of James McNeill Whistler, gen. eds Harriet K. Stratis and Martha Tedeschi, Chicago, 1998.