J'Onn was a male humanoid inhabitant of Nimbus III who lived in the 23rd century. J'Onn's life before Nimbus had been as a subject of Romulan rule in the Regulus system. At Regulus, J'Onn embezzled funds from his employer, and the punishment gave him a choice of exile or death. J'Onn was deported to become a settler on Nimbus III, along with his wife Zaara, who refused to denounce him.
On Nimbus, there was no hope. J'Onn and Zaara lived a meager existence searching for water in the desert. In 2287, Zaara died, and J'Onn resolved to suicide by working himself to death digging for water. It was while doing this that he encountered Sybok, on a quest to gain supporters for his Galactic Army of Light. Sybok used a Vulcan psionic technique, a form of mind meld, to help J'Onn resolve his grief, and gained his allegiance.
He was part of the force that kidnapped the Romulan, Klingon, and Terran ambassadors to Paradise City, the capital of Nimbus III. After luring the USS Enterprise-A to Nimbus III, he and the rest of Sybok's group stole the vessel to take it to the center of the galaxy, the fabled location of Sha Ka Ree. (TOS movie, novelization & comic adaptation: Star Trek V: The Final Frontier)