Waterhouse was a very private man and little is known about his models, other than family members. Additionally, it is believed that he often painted this version of an 'ideal' woman - an amalgamation of his favourite features and not a specific model.
The following links are very interesting and should give you a good deal of information as to who the candidates are:
(there are two videos on this website of use - if you look to the list on the right hand side -The Mysterious Models of John William Waterhouse, parts 1 and 2).
My own opinion is that the girl in Windflowers is most likely (given the dates) Ethel Bantock, his 'Echo' or possibly Mary Waterhouse (his Lady of Shalott) or Muriel Foster (his Lamia)...but it is hard to distinguish as he does give all his models a certain 'look'. Boreas, which has the same model features is believed to be one of Waterhouse's invented ideals, so Windflowers probably is too.