Mrs. Whatsit is one of the three immortal beings who help Meg, Calvin, and Charles Wallace throughout the whole story. Mrs. Whatsit can travel through time and space and, like other Mmes.
She was once a star that has sacrificed herself to destroy the Black Thing. In the image of a human being, Mrs. Whatsit is an old woman with mixed-up appearances. She was first found by Charles Wallace living in the haunted house together with Mrs. Who and Mrs. Which. Later on in the story, on the planet Uriel, Mrs. Which changes her appearance and becomes a centaur-like being. Among the three Mmes. W, Mrs. Whatsit is regarded as the most human-like being. At the same time, she has a mysterious power to see far beyond from what meets the eye.
As the characters realize how what they see in Mrs. Whatsit is just a small part of what she can be, her image serves for conveying the idea that appearances can be deceiving. Besides that, it helps to understand that there are things beyond human understanding, and the nature of Mrs. Whatsit is one of such things. Mrs. Whatsit is viewed as a supportive and warm old lady throughout the story. However, in the end, she seems a completely new person. She is distant and indifferent, and such a difference in her attitude to Meg and others serves for showing Meg once more that she can count only on her self.
Mrs. Whatsit in the Essays