Of the three Mrs. Ws, Mrs. Who seems the vaguest. Perhaps that's because even her words are not her own: nearly everything she says is a quotation from somewhere else. While Mrs. Whatsit has the strongest connection with the children and Mrs. Which is the most impressive personage, Mrs. Who's constant quoting reminds the kids (and us) that their efforts to roll back the darkness are part of an ongoing cultural project – and that A Wrinkle in Time's storytelling happens in the context of a literary tradition stretching back centuries. Mrs. Who's quoting may be a bit mystifying, but it also suggests a broader perspective both on events of the plot and on the novel itself.
Mrs. Who's other major characteristic is her glasses, which she gives to Meg as a gift on Camazotz. This gift perhaps suggests Meg's ability to see what the others cannot...or perhaps reinforces the idea that Meg's faults (like Mrs. Who, she wears glasses) are actually sources of power.