Petunia Monroe was Monterey Jack's former fiancee, from his days as Jack Swarthmore. She dumped him for the first of many skinny, blonde artist types and has been going through them with amazing regularity ever since.
They reunited briefly aboard the Tub O' Luv, forcing Jack to tell John about his priveliged upbringing. In a way, her breaking off the engagement was the best thing for the Dairy Revolution: it was a major factor in Jack's leaving the comforts of his family for military service and eventually the Revolution.
Petunia had thought her latest boy toy, Lenny, was to be the one true love of her life, but Jack threw him into the ocean where he was devoured by sharks.