Damar was a by-the-book soldier and looked down upon non-Cardassian races, particularly the Bajorans whom he had developed a hatred for after they had pushed the Cardassians off of the Bajoran homeworld. He especially despised Kira Nerys, whom he was forced to work with during the Dominion's occupation of Terok Nor. While he could be arrogant and cocky, the Dominion's rule of Cardassia caused him great distress: he had no real power and he turned to drink in order to forget his problems. Eventually, Damar came to realise that Cardassia was just another conquered planet under the Dominion flag and he found a new sense of purpose by trying to liberate his people from the Dominion's tyranny.
When he first appeared, Damar held the rank of Glinn and served under Gul Dukat on the freighter Groumall. When Dukat hijacked a Klingon Bird-of-Prey, Damar followed is mentor into a one-ship war against the Klingon Empire. He continued to serve as Dukat's aide when Cardassia joined the Dominion and during their occupation of Deep Space 9, during which time he was promoted to Gul. When the Dominion were driven off Deep Space 9, he murdered Dukat's daughter Ziyal for helping Major Kira escaping from confinement. Dukat refused to leave his daughter and Damar left him to return to Cardassia.
Following the Dominion withdrawal from DS9, Damar was promoted to Legate and was named leader of the Cardassian Union. However, Damar was nothing but a puppet of the Dominion and held no real power, his people's influence being subsidised by the Founders and their Vorta facilitators. Becoming increasingly disenfranchised caused Damar to turn to drink and he frequently brought bottles of kanar into Dominion Headquarters. As Weyoun and the Female Changeling continued to govern activities throughout the Cardassian Union without Damar's consent, he came to realise that the Cardassians were just another conquered people.
After the capture of Starfleet officers Worf and Ezri Dax in 2375, Damar decided to help them escape and told them to inform the Federation Alliance that they had an ally on Cardassia. He then went about organising a resistance movement against the Dominion and called for his people to rise up and fight for their freedom. Knowing that Damar was inexperienced with fighting a geurilla war, Starfleet sent Kira Nerys of the Bajoran Militia to assist in organising his troops by teaching them terrorist tactics.
Unfortunately, Damar's organised movement failed when they were betrayed by Dominion informants and most of his network was destroyed. Despite this loss, he had succeeded in rallying the Cardassian citizens into rising up against the Dominion and had begun a planetwide revolution that would greatly contribute to the Dominion's defeat.
Damar led a team of rebels, among them Kira and Garak, in an assault on Dominion Headquarters in the final day of the Dominion War. Shortly after making it inside the complex, however, Damar was shot repeatedly by Jem'Hadar soldiers. With his last breath, he urged his comrades to keep on fighting until they had won.