Who is Merricat?

   
"My name is Mary Katherine Blackwood. I am eighteen years old, and I live with my sister Constance. I have often thought that with any luck at all I could have been born a werewolf, because the two middle fingers on both my hands are the same length, but I have had to be content with what I had. I dislike washing myself, and dogs, and noise. I like my sister Constance, and Richard Plantagenent, and Amanita phalloides, the death-cup mushroom. Everyone else in my family is dead."

~ Shirley Jackson's We Have Always Lived in the Castle