Lemus was an Aquan male, native to the planet Argo. He was a Junior Tribune of the Ruling Tribunal of the Aquans.
When confronted by Spock and Kirk to assist in capturing a sur-snake, which was forbidden according to the Aquan ordainments, Lemus, along with Rila, disagreed with Nephro, who wished to follow his people's way. Lemus, however, believed that the Aquan ordainments were useless in times of turmoil, and that only the knowledge they could acquire, thanks to the assistance of Kirk and Spock, would help his people.
Once the trio were in agreement, they assisted Spock and James T. Kirk in capturing a sur-snake, so that the creature's venom could be extracted for use in creating an antitoxin that could reverse the effects of the surgo-op. (TAS: "The Ambergris Element")