Who is Frannie from Ahab's Wife, or, the Star-gazer: A Novel and what is their importance?

Frannie is the daughter of Aunt Agatha and Uncle Jonathan. She is Una's cousin and was Una's closest friend on the island. After leaving the lighthouse island, Frannie and her family move to a Great Lakes lighthouse. After Ahab's death, Frannie comes to live with Una, and then lives with Isaac at the Sankaty lighthouse and has a daughter by him. When the child drowns, she moves to Nantucket and works with Frederick Douglas.