During the Dominion War, Koval became an opponent of the Federation–Romulan Alliance, advocating that the Romulan Star Empire should seek a separate peace with the Dominion. This outspoken stance was believed to be the reason he had not been elevated to the Continuing Committee, as was usually the case for the chairman of the Tal Shiar. Koval often stated that there was a Federation spy in the Romulan Senate.
By 2375, Koval had been diagnosed with Tuvan Syndrome and shared this information with the Continuing Committee. According to Julian Bashir's diagnosis, the disease was in its very early stages, expecting a loss of motor skills in ten to fifteen years and an overall life expectancy of twenty to twenty-five years.
In that year, Koval attended a conference between the Federation and the Romulan Empire on Romulus. He approached Julian Bashir and asked him if the Quickening virus could be replicated and later attended a lecture that Bashir gave about the disease. It was at the lecture that Bashir noticed Koval's displaced eyelids, weakness in his facial muscles and irregular respiration, leading to his diagnosis of Tuvan Syndrome.
Later, and stalling for time while he believed Sloan was going to assassinate Koval, Bashir was given a microcellular adhesive to gain a skin sample from him, which could prove whether or not Koval had Tuvan syndrome. Bashir got the sample, but was detained and interrogated by Koval.
Koval later appeared before the Continuing Committee and told them that Section 31 did not exist, and that it had been fabricated by Sloan to seek revenge on Koval for the death of Fujisaki, without implicating the Federation as a whole.
After finishing delivering his findings to Praetor Neral, the Romulan leader told Sloan that he would be detained by the Tal Shiar. Sloan suddenly reached for a disruptor of a Romulan guard but was apparently vaporized by Koval.
William Ross later revealed to Bashir that Koval was a Federation operative and had been working with Starfleet Intelligence for more than a year. He was unaware of how long the Romulans had been working with Section 31. The "assassination plot" was in actuality a collaborative effort between Starfleet, Section 31 and Koval himself to ensure Kimara Cretak's dismissal from the Senate so that he would get his placement on the Continuing Committee instead of her. Koval's previous record of opposing the Federation Alliance would then serve to reinforce his arguments to keep the alliance intact. Koval's placement gave the Federation an ally inside the highest reaches of the Romulan government. (DS9: "Inter Arma Enim Silent Leges")
"It's a pleasure to meet you."
"Well, you've got me there. It's just a simple form of expression, I suppose."
"And like most Human expressions, completely devoid of meaning."
- - Julian Bashir and Koval
"What shall we talk about?"
"Why you're really on Romulus, who you're working for and, of course, who's helping you. Don't worry. I won't waste your time with pointless questions...This can be painful or not. That's up to you, Doctor. Either way, I will know what you know."
- - Julian Bashir and Koval, during an interrogation
"What I don't understand is why? Why did you come here and take the risk of being discovered?"
"I had to make sure nothing went wrong. And I wanted to watch you die."
"You broke the cardinal rule of our profession. You allowed business to become personal."
- - Koval and Luther Sloan
In the script for "Inter Arma Enim Silent Leges", Koval was described as, "a heavy-set Romulan wearing the black tunic of the Tal'Shiar, with a lot of iron-gray in his hair. His deeply lined face looks like it would shatter into pieces if he attempted a smile." 
John Fleck found that portraying Koval was challenging but he was thrilled to get through it, such as feeling a sense of achievement after performing a lengthy speech in a single camera take, which also delighted Director David Livingston. (Star Trek Monthly issue 91, p. 31) As it turned out, Koval became Fleck's favorite of the multiple Star Trek roles he played, until he appeared as Star Trek: Enterprise recurring character Silik. Fleck explained this was "because I had so much to do" in the role of Koval. (Star Trek Monthly issue 110, p. 32)
John Fleck's Koval costume, consisting of wood-grain patterned trousers with a matching jacket top and a black dry-deck belt that features an affixed gold painted resin Romulan Star Empire badge, was later sold off in the It's A Wrap! sale and auction on eBay for US$609.99. 
Koval appears in a number of non-canon works.
The Star Trek: The Lost Era novel The Art of the Impossible, reveals Koval was an operative of the Tal Shiar during the 2340s. In 2346, he learns of an interrogation conducted by Entek in which the Klingon Dirak reveals the House of Kultan is developing biogenic weapons on Khitomer. Koval later arranges a meeting with Jekri Kaleh, the head of the Tal Shiar. The story also states that, using the money and authority from his early role in the Tal Shiar, Koval is able to build a home at the summit of Kor Thorn on Romulus, but only his aide Timol and Jekri Kaleh know of its existence.
In the follow-up novel Catalyst of Sorrows, set in 2360, Koval holds the rank of Colonel and orders Crofter Thamnos and his family to develop a new strain of the Vulcan disease "Gnawing", as well as a cure for it. The novel also reveals that Kovel is in a romantic relationship with Kimara Cretak, who was of the same caste, and that Tuvan Syndrome runs in Koval's family.
Following the events of "Inter Arma Enim Silent Leges", the Vulcan's Soul novel Epiphany reveals Koval's Tuvan Syndrome has progressed to the point that he is seriously ill by 2377. However, in the Star Trek: Titan novel Taking Wing, he is assassinated in 2379, before the disease can kill him.
In the The Sky's the Limit short story Suicide Note, Koval is referred to as the general officer in the Tal Shiar leading the interrogation of Admiral Alidar Jarok's wife and daughter, following his defection, as featured in TNG: "The Defector". Jarok's farewell message reveals that Koval is actually one of Jarok's sympathizers, who have started to band together after the Continuing Committee had refused to address reports of increased Borg activity in the Delta Quadrant. Koval secretly pledges to keep Jarok's wife and daughter safe after his departure. Captain Jean-Luc Picard reactivates Koval as an agent for Federation Intelligence when he visits Romulus to deliver Jarok's farewell message, by which time Koval has been promoted to Vice Chairman of the Tal Shiar, replacing the recently-deceased Senator Vreenak.
Koval also appears in an alternate timeline featured in the novel Engines of Destiny, in which he holds the position of Deputy Arbiter under Supreme Arbiter of the Alliance, Sarek. Koval keeps a spartan office on Alliance Prime, from which he later contacts Sarek on an ultrasecure channel, in 2293. Koval informs him that three of the Cardassian members of the Alliance Council have a clandestine meeting, possibly to discuss overthrowing the Supreme Arbiter. Sarek then orders Koval to keep any Cardassian vessels in high orbit of Alliance Prime - out of transporter range - and to assign a bodyguard detail to Zarcot and any Cardassian Council member. Sarek later contacts Koval to brief him after Zarcot reappears, acting as bait in an attempt to destroy Sarek's escort vessel, the Wisdom. Sarek again contacts Koval as he orders the Alliance fleet to intercept a phalanx of Borg cubes that are intent on destroying the USS Enterprise-D.