Thérèse is an elderly Caribbean woman who does laundry for Valerian Street. She is the aunt of Gideon or "Yardman" as the wealthy Street family likes to call him. The Streets call her "Mary" because they don't know any of the local women's real names. But Thérèse isn't going to just sit around and be stepped on by these white people and their black American servants. In fact, she goes out of her way to never acknowledge these people when they're in her presence:
"Unlike Thérèse whose hatreds were complex and passionate as exemplified by her refusal to speak to the American N****es, and never even to acknowledge the presence of the white Americans in her world" (4.217).
Thérèse has lost most of her eyesight, but is able to compensate for it by being sharp in other respects. Her uncanny sense of smell, for example, allows her to detect Son's presence on Isle des Chevaliers days before anyone else knows he's there: "She caught the scent twelve days ago: the smell of a fasting, or starving, as the case might be, human" (4.188).
For all her bitterness against the Streets and their servants, Thérèse really likes Son. He's like an exotic outsider from America who actually shows interest for Thérèse and her life. As the book says: "She had seen [Son] in a dream smiling at her as he rode away wet and naked on a stallion" (4.187). Thérèse likes parading Son around in her house like a trophy for her neighbors to stare at and meet.
At the end of the novel, Thérèse is the only person who's willing to take Son out to Isle des Chevaliers when he comes looking for his (ex) girlfriend Jade. Thérèse risks a lot by making the dangerous boat crossing, especially considering that she's blind.