Demmas was the eldest son and heir of the Autarch of the planet 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; however, Tieran was determined to overthrow Demmas' family and reinstate himself as ruler. Demmas 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.