Who is Joanna Dover from Women and what is their importance?

Joanna Dover is described as extremely tall, stately, proper, and very rich. She is also highly educated and apparently successful in business. Joanna meets Henry through a common friend and purchases a few of his paintings. After which, they have sex. Several weeks later, Henry visits Joanna in Galveston, Texas, where they spend a week together drinking, using drugs, and having sex.