The Xebellians were originally the inhabitants of an otherworldly realm called Dimension Aqua. In the modern era, they are the descendants of vanquished exiles defeated in an Atlantean civil war. Most Xebellians have some form of hydrokinesis, which appears to be a rare trait in Atlantis.
In recent history, King Ryus of Xebel had two daughters - Mera and Hila - but no sons. Accordingly, it was decided that his heir would be the man who married his eldest daughter Mera. Mera was betrothed to Nereus, the leader of the Xebellian military, but after she defected to Atlantis, Nereus ascended to the throne of Xebel after Ryus as a king without a queen.