Mathilde (more commonly known as Hildy) is the wife of Tam. Hildy built the bridge that Todd and Viola crossed to get away from Prentisstown. They are the first people that Todd and Viola meet after leaving the swamp and Hildy is also the second female he had ever remembered meeting. Todd mistakes her for a man when they first meet and is confused about the lack of Noise.
She and Tam came to New World on the first settler ship. Their house is made from the ship that the came down in, a Expansion X Three 200 that they remodeled to look like a swan. When talking about their home, Hildy mentions that she is or was an engineer. Hildy has lived on Old World so her parents weren't caretakers on the settler ship. She was old enough to remember what it was like back on Old World.
She owned the bridge Todd and Viola blew up in The Knife of Never Letting Go, but still takes them in and feeds them and lets them rest. Todd doesn't trust her at first but Viola always did. She takes them into the town and introduces them to her sister, and they deputy mayor. She is obviously a figure of authority and people listen to her. When the Prentisstown men attack, she runs back to her house because Tam was still there. She is presumed dead.