Who was Martha Washington? How was she Helen's good companion?

Martha Washington was the daughter of Helen Keller’s family cook and was her friend and playmate when she was younger.
When Helen was little, Martha Washington was her playmate but hardly her equal. 

In those days a little coloured girl, Martha Washington, the child of our cook, and Belle, an old setter, and a great hunter in her day, were my constant companions. Martha Washington understood my signs, and I seldom had any difficulty in making her do just as I wished. (Ch. 2)

Because Helen was blind and deaf, she liked the fact that Martha Washington could understand her signs.  Young Helen found it very hard to communicate with anyone, so this was a big benefit.  The other advantage was that Martha let her push her around.  Young Helen was a bit bossy.  The fact that Helen mentions her at the same time as the dog shows that she considered her more of a companion than an equal.
In addition to the race difference, Helen Keller notes there was also an age difference between the two girls.

Martha Washington had as great a love of mischief as I. Two little children were seated on the veranda steps one hot July afternoon. ... One child was six years old, the other two or three years older. The younger child was blind–that was I–and the other was Martha Washington. (Ch. 2)

It seems like Martha Washington enjoyed getting into trouble with Helen Keller though.  Little Helen was very intelligent, and even though she could not speak like a normal person or hear what Martha was saying, they could still play together and be very creative in getting into trouble.
Once Helen started working with Anne Sullivan, she learned sign language and was able to communicate more fully with more people.  She was able to learn a more standard language.  Her early childhood was pretty lonely though.