Ginnie Hempstock is not exactly your typical mom—she's got powers and knowledge that would blow June Cleaver and her vacuum out of the water. And we're not even really sure that she's technically Lettie's mom, at least in the biological sense:
"She didn't have a daddy?" I asked.
"Did you have a daddy?"
"You're all questions, aren't you? No, love. We never went in for that sort of thing. You only need men if you want to breed more men." (15.11)
Blood relation or not though, she calls Lettie her daughter, and when Lettie sacrifices herself for the boy, she's devastated, so we're sticking with the mom descriptor. She even has the ruddy cheeks and ample bosom that we like to envision for stereotypical English country mums.
Other than being a maternal figure, though, we don't know all that much about her. She's quick with a lie to cover for their supernatural activities (she's the one who came up with the Lettie-went-to-Australia falsehood), and she's awfully fond of wellingtons.