Demmas was the eldest son and heir of the Autarch of Ilari. He had a younger brother, Ameron.
In 2373, Tieran, a tyrant who could transport his mind into someone else's body and had survived over two centuries, took control of Kes' body and escaped from the USS Voyager. Demmas visited Voyager and explained Tieran's nature to the senior officers. He explained that Tieran had been overthrown by his family's ancestors over two hundred years prior, after which his family became the new imperial house and rulers of Ilari, and Tieran was determined to overthrow his family and reinstated himself as ruler. He also informed the crew of Voyager about Tieran's ability to transfer his consciousness from one body to another, telling them that Kes was lost forever.
While Demmas was on board Voyager, Tieran and his followers invaded the palace and killed the Autarch. He disagreed with Captain Janeway's decision to try and rescue Kes, and wanted her to use Voyager to destroy Tieran and his followers. Demmas and the Voyager crew were able to depose Tieran and save Kes. Demmas then became the Autarch. (VOY: "Warlord")