Who was Martha Washington? What sort of relationship did Helen have with her?

Martha Washington was Helen Keller's youthful companion. Helen never calls her a friend.
Martha is a nine-year-old black girl, the daughter of the Keller family's cook. She, Helen, and the dog play together. Although Helen Keller is roughly three years younger than Martha, she dominates the relationship:

Martha Washington understood my signs, and I seldom had any difficulty in making her do just as I wished. It pleased me to domineer over her, and she generally submitted to my tyranny rather than risk a hand-to-hand encounter.

Together, Martha and Helen play in the kitchen and feed the hens and turkeys that come to the kitchen steps. They also get into mischief together. For example, they steal a frosted cake from the kitchen, take it to wood pile, and eat it. Sitting on the porch one day, they cut off everything within reach, including each other's hair.
Though Keller doesn't state this, it's possible Martha was assigned with the job of keeping her energetic self out of the way of the adults who had to get things done. Keller also never seems to consider that the older girl submits to her because she has been taught that whites are in charge.