Who is Castiglia from Oldest Living Confederate Widow Tells All and what is their importance?

Castiglia (Cassie) was Lady Marsden's slave. After the war, she went to work for Will. Cassie worked six and one half days a week and lived in a cottage Will purchased for her seven blocks away. He eventually fired Cassie after learning that Lucy was seen in Baby Africa. Cassie had six children but never married. She remained Lucy's life long friend.