The real-life inspiration for Mary Poppins: Aunt Sass.

From the Guardian:

Travers called her great aunt Ellie, whose real name was Helen Morehead, Christina Saraset, or Sass for short. In her semi-autobiographical story Aunt Sass, she writes of how "her remarkableness lay in the extraordinary and, to me, enchanting discrepancy between her external behaviour and her inner self", adding "imagine a bulldog whose ferocious exterior covers a heart tender to the point of sentimentality and you have Christina Saraset".