Gard was a Trill symbiont who, over the centuries, consistently served as a special agent of the Trill Symbiosis Commission in seeking out the most serious threats to Trill society.
Gard had had many more hosts than any other symbiont, and had also spent far longer than the usual several centuries as a joined Trill. The late-24th century host, Hiziki, remarked that he couldn't remember all of the previous hosts or their actions. He didn't know if this was because there were so many, or if it was due to some security measure he hadn't been briefed about. However, he didn't care as his purpose was all that mattered to him. (DS9 - Worlds of Star Trek: Deep Space Nine novel: Trill: Unjoined)
One of Gard's specific concerns was dealing with the rare joined Trill who were so unstable as to become murderers. The first such "monster" was known as "The Butcher of Balin"; the fourth was executed by Kirista Gard circa 2195. The fifth, Joran Dax, was killed by an intended victim, but Verjyl Gard was present to help save the Dax symbiont. (DS9 - The Lives of Dax short story: "Allegro Ouroboros in D Minor")
Because of this specific purpose, the hosts selected for Gard underwent an even more rigorous screening process than for normal initiates. The potential host would be told about Gard's purpose and allowed to make an informed decision about whether to go ahead with the joining. As of 2376, only one selected host, a female, had refused. (DS9 - Worlds of Star Trek: Deep Space Nine novel: Trill: Unjoined)
Circa 2356, Verjyl Gard was approached by Taulin Cyl and informed about the Parasites, and the potential threat they posed to Trill. Gard became part of an unofficial group of Watchers who kept vigil for the creatures' return. In 2376, it was suspected that First Minister of Bajor Shakaar Edon had come under influence of the Parasites, and Hiziki Gard was assigned to a Trill diplomatic team to Deep Space 9. Upon determining Shakaar was indeed irreversibly infected, Gard assassinated the Bajoran leader.
Shortly thereafter, Gard volunteered to be joined with a "mother" Parasite, in order to determine the extent of their infiltration on Bajor. He was able to obtain vital information before the Parasite was removed from him, with no permanent damage done. With the Parasite threat publicly acknowledged and defeated, Hiziki Gard was extradited to Trill and pardoned. (DS9 novel: Unity)
Gard was present during the neo-Purist incited riots in early October 2376. He worked with Taulin Cyl, Ezri Dax and Julian Bashir during the crisis. (DS9 - Worlds of Star Trek: Deep Space Nine novel: Trill: Unjoined)