Nephro was an Aquan male, native to the planet Argo. He served on the Junior Tribune of the Ruling Tribunal of the Aquans.
He was approached by James T. Kirk and Spock to help them capture a sur-snake. Nephro was cautious, as he feared Domar's warning that if the Junior Tribune broke any more ordainments, they would be exiled to the open seas. In spite his wishing to heed Domar's words, he found himself in disagreement with Lemus and Rila, who eventually convinced him to help Kirk and Spock.
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 anti-toxin that could reverse the effects of the surgo-op.