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

Frannie is Aunt Agatha and Uncle Jonathan's daughter. When Frannie and her family leave the lighthouse island, they move to a Great Lakes lighthouse. After Ahab's death, Frannie goes to live with Una, after which, she lives with Isaac at the Sankaty lighthouse. Frannie and isaac have a daughter together.