Martha Sowerby is our introduction to The Secret Garden's super-positive portrayal of the poor rural residents of England: She is rosy-cheeked, hard-working, no-nonsense, family-oriented, and generous. She is also Mary's maid at Misselthwaite Manor. As soon as Mary sees Martha, Mary realizes she won't be able to get away with the bullying and physical violence that Mary used to turn on her Indian nannies.
But while Martha won't tolerate tantrums and expects Mary to get dressed on her own like the capable kid that she is, she is also very kind: Martha sees Mary's interest in gardening and offers to bring her some tools. She also introduces Mary to her little brother Dickon, who basically saves Mary's life, or at least her soul, with his generous help in reviving the Secret Garden.
Honestly, instead of being a real, three-dimensional character, Martha is mostly a plot device for Mary's coming-of-age narrative. She introduces Mary to the story of the Secret Garden and to Dickon, providing the first step to Mary's larger change of heart. But since Martha is mainly in the novel to get Mary out of her own head and into the great outdoors, there isn't a lot to her as an independent character.