Elim Garak (portrayed by Andrew Robinson) was a semi-regular guest character from Star Trek: Deep Space Nine. He was once a member of the Cardassian Union's central intelligence bureau, the Obsidian Order, operating as a spy and saboteur. Exiled from his homeworld of Cardassia Prime after disregarding his duties during the Cardassian Occupation of Bajor, Garak was forced to reside on the space station of Terok Nor (renamed Deep Space 9) when the United Federation of Planets took control of the station at Bajor's request. While Garak found it difficult living amongst Bajorans who hated him and his kind, he managed to make a life for himself by opening up a tailor shop on DS9's promenade and even forged a friendship with Starfleet doctor Julian Bashir. He still maintained several contacts on Cardassia and continued observing and reporting on the various activities on DS9 that may be of interest to his people.
Not Quite Dead...
While Garak survives the entire DS9 series, he appears to die in the third season episode "The Search - Part Two". When Sisko and several of his crew try to hijack a runabout in order to destroy the Bajoran wormhole so as to stop the Dominion from gaining a foothold in the Alpha Quadrant, Garak helps them get past several Jem'Hadar soldiers on the way to the landing pad. Garak appears to get shot by one of the soldiers, but it turns out that Sisko and his crew weren't even on Deep Space 9, but on the Founders' home planet hooked up to some sort of virtual reality interface. The Garak that appeared to be shot wasn't real and the real Garak hadn't even been present during the crew's mission to find the Founders.
In the mirror universe, Garak is a high-ranking official in the Klingon-Cardassian Alliance. He reported directly to Indendant Kira Nerys before Terok Nor was captured by the Terran Resistance and then to Regent Worf afterwards. Like his prime counterpart, he was a highly skilled interrogator and saboteur, though he also took a sadistic pleasure from making others suffer.
In DS9's final mirror universe episode "The Emperor's New Cloak", Garak is tasked with disposing of Quark, Rom and Grand Nagus Zek. He selects a hypospray loaded with a concentrated dose of ulcartic virus as his method of execution. The virus was said to deliver an excrutiatingly painful death by raising the victim's internal body temperature so high that the victim's organs would literally boil. Before he could deliver the virus to Quark, however, Ezri Tigan got the drop on Garak and snatched the hypospray, then injected him with the virus. She and the Ferengi then made their escape and left Garak for dead.