Mirren Hadar was a Cardassian civilian engineer stationed at a Ketracel white manufacturing plant. He used his position to provide the technology involved in synthesizing ketracel white to agents of the Romulan Star Empire's Tal Shiar. Hadar would also assist mercenaries hired by the Tal Shiar to raid Starfleet ships for the latest technological innovations in order to assess which side in the Dominion War was a bigger threat to Romulan interests.
During a raid on the USS Lambda Paz, in which Nausicaan, Breen, and Ferengi mercenaries stole most the ship's antimatter tanks and a large number of gel packs, Hadar was captured by Lambda Paz personnel. He was very uncooperative when Captain Limis Vircona attempted to interrogate him.
After the Lambda Paz retrieved most of the stolen equipment, away teams also stumbled across the technology involved in synthesizing ketracel-white. Limis interrogated Hadar again, this time by placing him in an airlock and decompressing it. Hadar provided the location of the manufacturing plant.
Ironically, he died immediately afterward when sudden re-exposure to the ship's atmosphere caused fatal cardiopulmonary damage. Chief medical officer Aurellan Markalis likened the cause of death to frostbite, where the affected area needs to be warmed slowly, otherwise the sudden blood flow could, at worst, lead to potentially fatal organ damage. In Hadar's case, his sudden re-exposure to atmosphere proved fatal as the air pressure calibrations on Starfleet vessels was not ideal to Cardassian physiology. (Star Trek: Lambda Paz: Moral Dilemma)
For the next two and a half years, Mirren's brother attempted to exact vengeance on Limis for the murder of an innocent civilian. By mid-2376, Gul Enic Hadar, with the help of True Way leader Gul Revok, had succeeded capturing Limis. His hope was to use her as a bargaining chip with the government. (Star Trek: War Aftermath: Episode 1: The True Way)