Mrs. Whatsit is one of the characters of the A Wrinkle In Time novel.
She is first described as a very old woman wrapped in layers of clothes and first appears in chapter one. Charles Wallace, a five-year-old boy in the book, found her in a 'haunted house' in the woods, where she has been living with her two friends, Mrs. Who and Mrs. Which. Mrs. Whatsit is the youngest of the Mrs. Witches (despite being over 2 billion years old), and the best of the three at interacting with the children.
In Chapter Four, the group (Charles, Calvin, and Meg) witnesses the physical transformation of Mrs. Whatsit into a centaur-like winged being on the planet Uriel. Mrs. Whatsit is also revealed to have been a star that sacrificed itself by exploding in order to destroy a section of the Dark Thing.
In chapter 6, Mrs. Whatsit admits that she used to be a star, but sacrificed herself to try to overpower the Darkness.
In the movie, Mrs. Whatsit is a tall woman that has red hair and fair skin and wears a white dress.