The manager of a store in the town of Ferne, Mattie has a "plump face" (3.9) and she is a friend to Micah's family. When the old magician lady is trying to haggle with Micah for a better price to help his mom, Mattie says: "That boy's mother is my friend. Hurt her and I'll have your ears" (3.14). Too bad she isn't able to carry out that threat when the magician lets Micah's mom die and sacks their house.
Something must've happened to her at some point, though we don't know what, since she has an "uneven gait" (9.35). When Sadima grows up a bit, Mattie keeps asking her to come visit, but Sadima's dad is over-protective and doesn't allow it. Still, whenever Mattie sees Sadima, she reminds her: "If you ever lack anything, you know where to come… I am your mother if you ever need one" (9.42), which is both really sweet and shows how dedicated she is to Sadima's family.
Mattie remains in Sadima's thoughts even after she's been living in Limòri for a while. When stuff with Somiss starts getting weird, Sadima thinks about going back to Ferne: "Mattie would take her in. Then she would find a way to open a cheese shop in town" (45.9). Mattie represents a normal life to Sadima—a simple life in the countryside, completely non-magical, surrounded by a loving family.