Aamin Marritza was the head file clerk at the infamous Gallitep labor camp on Bajor under Gul Darhe'el. While at Gallitep, Darhe'el praised Marritza's work as a "masterpiece of meticulous exactitude," but Marritza took no pride in his commander's praise; indeed, Marritza was so overcome with grief upon hearing the screaming and begging for mercy by the Bajorans at the camp that he would often hide under his desk, covering his ears and weeping. He served at the camp until its liberation in 2357.
Following the Cardassian withdrawal from Bajor, Marritza relocated to Kora II where he became an instructor in filing at the Cardassian Military Academy. While on Kora II, he underwent cosmetic surgery to resemble Gul Darhe'el. His plan was to convince the Bajorans that he was Darhe'el so they would convict and punish him as a war criminal as a way to force Cardassia to acknowledge the atrocities committed during the Occupation of Bajor. He was was consumed by guilt concerning his service to Darhe'el and because he did nothing to help the Bajorans, to the point that he was willing to die to absolve himself and Cardassia of the sins of the occupation.
He traveled to Deep Space 9 in order to be exposed as Darhe'el. He had a rare disease, Kalla-Nohra Syndrome, for which the only cases were the result of a mining accident at the Gallitep forced labor camp. He claimed to be Marritza in order to convince the Bajorans that he was really Darhe'el posing as Marritza. His plan was almost successful. He convinced Kira Nerys that he was the 'butcher' of Gallitep, clouding her judgment by boasting of the atrocities at the labor camp and filling her with so much rage that she wanted him to be Darhe'el, so that he could be tried and executed for his crimes.
Marritza's deception and plan was unraveled by Odo, who discovered Marritza had been seeking information about Kira months previously. Additionally, Odo found out that he put all of his personal affairs in order, resigned his professional position and provided handsomely for his housekeeper before his departure. He also learned of Darhe'el's death from Gul Dukat, who crucially informed him that Darhe'el hadn't been on Bajor when the mining accident that caused the Kalla-Nohra condition occurred by providing him with official records to validate these assertions. In addition, Doctor Bashir found that Marritza had his face changed to look like Gul Darhe'el and was taking massive doses of a dermatiraelian plastiscine, a dermal regenerative agent. Kira promptly confronted him and gently brought the truth out, making him weep and admit the horror and grief he felt about what he had experienced and his sense of personal shame at not stopping the atrocities that had happened. Even with all the evidence against him, Marritza asked Kira to help him complete his plan so that Cardassia could face up to its evils. Kira, humbled by this revelation, was unwilling to be complicit in the execution of an innocent man, instead reflecting that Cardassia would be better if it had more men like him willing to face what his people had done.
He was released, only to be fatally stabbed on the promenade by a racist Bajoran named Kainon; not because he was Darhe'el, but because he was a Cardassian. His death was mourned by Kira. (DS9: "Duet")
The script for "Duet" provides the pronunciation of Marritza's name as "A-min marr-IT-sa". 
Writer Peter Allan Fields was delighted that Yulin was cast in the role as he had been a fan of the actor for many years. "I'd always wanted to write for him," Fields explained, "I was delighted that he wanted to do it." Executive producer Ira Steven Behr echoed Fields' comments and gave some insight into the development of his character; "We would go into those long monologues and stand and rant and scream, and actually a lot of it, word for word, is in there. Cardassians love to speak. Garak loves to speak, Enabran Tain loves to speak. Dukat loves to speak -very slowly- and certainly Marritza loves to speak." (Star Trek: Deep Space Nine Companion, p. 65)
Director James L. Conway thought Yulin, "did a fantastic job. However, the makeup was three hours in morning and two hours at night and he had to get there at four in the morning. He was so tired that he had a very hard time remembering the lines, and we had a very hard time getting the performance. Then, it was one those performances where you don’t really know you have it until it’s cut together. He also didn’t like the ending. He got so invested in his character that he didn’t want his character to die at the end. He was trying to justify it and have him somehow survive, which almost never happens on a television show. The guest star never starts trying to change a script. He did, but mostly because he was so invested in the character. But it was one of those shows that, until it was edited, I didn’t realize how good it was." 
Marritza also appears in the Star Trek: Terok Nor novel Night of the Wolves, in which it reveals he was unwittingly responsible for the liberation of Gallitep by hinting to a Cardassian scientist, sent to oversee the shutdown of the camp's computer system in preparation for the camp's closure, that the Bajoran workers would likely be killed once their usefulness had ended. This scientist, a Bajoran informant, passes the information on to the Shakaar resistance cell, which motivates them to liberate the camp.
In the novel Warpath, Kira is stabbed by the Jem'Hadar Taran'atar. Being close to death, she remembers Marritza's senseless murder and being with him when he died. Later, during an encounter she has with the Prophets, one takes the visage of Marritza.