Mrs. Earwig (pronounced "Ah-widge") is an arrogant, snobbish witch. She is married to a retired wizard Dr Earwig, and is of the kind of witches who read books, and has even written several books on "magik". Her last, and only, trainee was Annagramma Hawkin.
Although she does use practical folk wisdom as a witch, she also tends to use crystals, jewelry and other items considered useless by witches of Esmeralda Weatherwax's tradition. (Not that Granny doesn't recognise the uses of Boffo, but you don't have to go believing it yourself.) Needless to say these two are intense rivals, and treat each other with hostile politeness.
She does however help promote the craft among younger girls by selling her various books including First Flights in Witchcraft, My Fairy Friends, and To Ride a Golden Broomstick, in addition to an unnamed book on "Flower Magick". These are widely denounced by other witches as being ridiculous nonsense that only appeals to those who don't know what witchcraft is really about.
Her husband Dr. Earwig is mentioned in Unseen Academicals as one of the few wizards who have retired (although he technically just left to get married, which, according to Mustrum Ridcully, is the same as dying).
Her complete self-absorption does come in handy. In The Shepherd's Crown, she is shown to be completely immune to Elven Glamour, which leverages human self-doubt. With Letice, there's nowhere for such a lever to gain a purchase.