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

Bertha Spenser is the wife of Ulysses and the mother of Una. She sends her daughter to live with her sister at the lighthouse and remains in Kentucky. Una arrives to await the birth of her baby. Bertha is killed in a carriage accident on her way to call for the doctor when Una is ready to deliver.