Tribune Rila was an Aquan female, native to the planet Argo. She was a Junior Tribune of the Ruling Tribunal of the Aquans.
Rila interceded in favor of Kirk and Spock at the hearing, alerted the rescue party when they were returned to the surface to die.
Rila, Lemus and Nephro agrees to help them if they will in turn help her find a buried ancient city in ruins whose archives may contain a way to reverse the mutation. They discover that the substance in their bloodstreams is similar to the ambergris of Earth whales. The reverse surgo-op mutation is induced by an infusion of an anti-toxin made from the venom of a deadly giant sur-snake. A process also effective to younger aquans which want to return to the surface after reverse their water-breathing mutations.
Rila was present on the bridge of the USS Enterprise when phasers fire were used to stabilized seismic activity of the Argo.