In the novel The Mirror Crack'd from Side to Side, Miss Knight is a woman sent by Raymond West, to take care of his aunt after a period of illness.
Miss Knight is described as a "big, rather flabby woman of fifty-six with yellowing grey hair very elaborately arranged, glasses, a long thin nose, and below a good-natured mouth and a weak chin". She is conscientious and kind, but quite maddening to live with. Miss Marple feels that such people are "full of kindness, ready to feel affection towards their charges, to humour them, to be bright and cheerful with them and in general to treat them as slightly mentally afflicted children". She is also annoyed by Miss Knight's habit of referring to her as "we", saying things like, "I hope we've had our little snooze?".
Happily for Miss Marple, Miss Knight receives a letter from Lady Conway, saying how anxious she is for Miss Knight to join her at the Belgrave Hotel at Llandudno. Miss Knight says that she would never dream of not fulfilling her obligations to Miss Marple, and so does not intend to go. However, after Cherry Baker asks if she and her husband can stay in the rooms above the kitchen in Miss Marple's house, Miss Marple sees it as an opportunity for Miss Knight to go to Lady Conway.
Miss Knight reminded Miss Marple of Emily Waters, as she has just "the same bird brain".