King Gustav was the hereditary monarch of Kaznia.
Gustav gained immense world popularity through active contributions to an International Space Station, but at the cost of economic stability at home. He also seems to have taken a harsh, authoritative approach to his reign that alienated many of his subjects.
After arranging the marriage of his daughter, Audrey, to Vandal Savage, the space program's administrator, Gustav was poisoned by one of Savage's agents disguised as a chambermaid, becoming paralyzed by an apparent stroke. With him incapacitated, Audrey assumed the throne.
When the royal palace was destroyed in the Justice League's fight to stop Savage's plot, Gustav was among those carried out in the evacuation. It is unknown whether he ever recovered from his poisoning, or if he died when the palace was destroyed, but at a later date, Audrey was still shown to be ruling Kaznia.