Mrs. Whatsit is the most human-like of the three Mrs. Ws (at least until she turns into a freaking winged centaur), but there's always more to her than meets the eye. When she first blows into the Murry house, Meg thinks she looks like a tramp
The age or sex was impossible to tell, for it was completely bundled up in clothes. Several scarves of assorted colors were tied about the head, and a man's felt hat perched atop. A shocking pink stole was knotted about a rough overcoat, and black rubber boots covered the feet. [...] Under all this a sparse quantity of grayish hair was tied in a small but tidy knot on top of her head. Her eyes were bright, her nose a round, soft blob, her mouth puckered like an autumn apple. (1.104, 110)
Mrs. Whatsit's mixed-up appearance suggests someone who's gone through the free bin at a garage sale – it's made up of odds and ends, as if she just grabbed whatever was nearest to hand. Underneath her mismatched exterior, however, Mrs. Whatsit looks like everyone's favorite grandmother. We find out later that Mrs. Whatsit could take pretty much any form she wanted, so why is her style choice hobo grandma? Perhaps she doesn't have a keen enough eye for human fashion to realize how strange she looks to many eyes, or perhaps she likes looking a little off-kilter.
Even in this inauspicious outfit, however, Mrs. Whatsit is clearly more than the tramp she appears to be. Her parting shot to Mrs. Murry – "Speaking of ways, pet, by the way, there is such a thing as a tesseract." (1.51) – turns the usually composed woman white. It's plain to see that Mrs. Whatsit knows more than she's saying, and that, like the Terminator, she'll be back.
Once Meg, Charles Wallace, and Calvin start traveling with Mrs. Whatsit and her companions, further information from her bio is revealed in dribs and drabs. After hearing Mrs. Which rebuke Mrs. Whatsit repeatedly for showing the follies of youth, Calvin asks Mrs. Whatsit just how old she is. Her answer? "Exactly 2,379,152,497 years, 8 months, and 3 days. That is according to your calendar, of course, which even you know isn't very accurate" (5.76). Finding out that the Mrs. Ws' timescale is such that 2 billion plus years is a short-ish time clues us in that they're working in an entirely different frame of reference from the human one (if the fact that Mrs. Whatsit turned into a WINGED CENTAUR didn't already give that away).
All these hints that there's more to Mrs. Whatsit than meets the eye lead up to the revelation that she was once a star, but sacrificed herself in the war against the Black Thing. Her human face is just a facade:
The complete, the true Mrs. Whatsit, Meg realized, was beyond human understanding. What she saw was only the game Mrs. Whatsit was playing; it was an amusing and charming game, a game full of both laughter and comfort, but it was only the tiniest facet of all the things Mrs. Whatsit could be. (6.17)
From Meg's limited human perspective, Mrs. Whatsit is a creature of infinite possibility: Meg can't even begin to imagine the entire extent of her being. While this mysterious grandeur might be awe-inspiring, Mrs. Whatsit doesn't stand on ceremony: the fact that she, and the other Mrs. Ws, play their little fashion games in the midst of an epic battle of good vs. evil suggests that it's when you lose your sense of fun that the bad guys truly win.
When Mrs. Whatsit returns after the Murry-O'Keefe adventures on Camazotz, she seems an entirely different character:
Appallingly, Mrs. Whatsit's voice was cold. "And what do you expect us to do?" (12.6)
The once warm and supportive Mrs. Whatsit is now cold and distant, and her current manner is all the more disturbing in its about-face from her past affectionateness. Why the sudden shift? It's hard to say, but it seems to have something to do with Meg's expectation that Mrs. Whatsit and her companions will sweep in and clean up the mess of Charles Wallace's succumbing to IT. Mrs. Whatsit's stone cold attitude is some tough love, to get Meg to stop looking for a knight on a white horse (or winged centaur) to save the day, and to do what she's got to do – and to be a grown-up about it.
"We want nothing from you that you do without grace," Mrs. Whatsit said, "or that you do without understanding." (12.31)
It seems Mrs. Whatsit's coldness is all part of a cunning plan to get Meg to realize that she can't count on others to solve her problems – and that this is something she can't be told, but has to figure out for herself, of her own free will.
Mrs. Whatsit does, however, as befits her role as mentor, give Meg a pep talk before she sends her off to do battle with the brain dude for her brother's soul.
"We gave you gifts the last time we took you to Camazotz. We will not let you go empty handed this time. But what we can give you now is nothing you can touch with your hands. I give you my love, Meg. Never forget that. My love always." (12.96)
Mrs. Whatsit's affirmation of love gives Meg the spark she needs to save Charles Wallace. By making Mrs. Whatsit's last and strongest message be one of love, the novel suggests that, whether you're a puny human child or a grand ex-star, love can transcend boundaries to link everyone, no matter how different.