"But not anymore?"
"As I've grown older, I realize that clarity of purpose is a more ambiguous matter than I had thought in my youth."
At the time of his defection, DeSeve held the Starfleet rank of Ensign. He was motivated by his belief that the Romulans had clear moral values and an absolute certainty about the purpose of life, something he referred to as a "strict moral compass". However, after living on Romulus for twenty years, he became disillusioned with their way of life. The Romulan desire for conquest and the deception employed to that end did not sit well with him. (TNG: "Face of the Enemy")
Return to the Federation
By the late 2360s, DeSeve began to sympathize with the Romulan underground movement, and decided, in part, to return to the Federation in order to deliver a message from Ambassador Spock. After traveling for two weeks through Romulan space aboard a scout ship he finally arrived back to the Federation.
Following his arrest, DeSeve insisted on speaking to Captain Jean-Luc Picard on an urgent matter. Picard agreed to meet with him, and relayed the message from Spock, explaining that a Corvallen freighter would arrive shortly in the Kaleb sector, carrying an important cargo that was to be transported back into Federation space. Picard considered DeSeve's request, after making a request of his own, to uncover DeSeve's motives for defecting back to the Federation. After due consideration, Picard ultimately agreed to DeSeve's request and set a course for the Kaleb sector.
Unknown at the time, however, the Corvallen freighter, which was transporting the cargo, was destroyed during its rendezvous with the Romulan warbird IRW Khazara. When the freighter did not arrive at its designated coordinates, Picard became suspicious of DeSeve once again, who himself did not understand why the freighter had not arrived.
Picard pried DeSeve for more information, only to learn that DeSeve had neglected to mention the type of ship to expect, what the cargo was, nor that he had not in fact received his information directly from Spock as he had earlier alluded to, but from a third party. DeSeve explained that, at the time, the information "didn't seem necessary," and that on "Romulus you learn not to volunteer information," describing it as "a hard habit to break." Lieutenant Worf expressed his dislike for DeSeve suggesting that the defector may be leading the crew into a trap. Fortunately for DeSeve, Picard elected to continue with the mission.
After efforts to locate the freighter resulted in the discovery of its debris, and an anomalous polarized magnetic distortion. DeSeve speculated that the distortion may very well be a cloaked Warbird, and once the anomaly was discovered to be fact, revealed what he knew of Romulan tactics in these types of situations.
When the Warbird finally revealed itself to the Enterprise, and transported three Romulans to the bridge via a disruptor beam, DeSeve was able to quickly identify the "cargo" as Romulan Vice-proconsul M'ret and his two aides. (TNG: "Face of the Enemy")
DeSeve was played by actor Barry Lynch.
According to the script of "Face of the Enemy" his first name was Stefan. The script also described DeSeve as, "in his late forties, but his face is lined and worn beyond his years. He wears the unadorned uniform of an enlisted Romulan soldier." It goes on to describe his introductory scene as, "The moment he sees Riker, he comes stiffly to attention. Riker reacts to the sight of the uniform; he doesn't like seeing DeSeve in it."
Several of DeSeve's lines were also cut from the finished episode, notably a scene in which he attempts to order a cup of Vorcha'ak before realizing he should say "coffee", references to him being disoriented and alone after his return to the Federation and a line in which he apologizes to Captain Picard for referring to him as "Commander", stating, "Captains, not Commanders... it's been so long since I've... I'll remember." (Star Trek: The Next Generation Companion (2nd ed., p. 234) ; )
DeSeve's mention of the term "cowboy diplomacy" is a reference to Picard's use of the term during conversation with Spock in the episode "Unification II". He is also the first character to mention the Romulan use of artificial quantum singularities on their ships, a point that will be picked up in a number of future episodes.
In his review of "Face of the Enemy", Star Trek author Keith R.A. DeCandido praised the character, writing, "What I especially admire about this episode is the way that DeSeve’s internal conflict mirrors that of the episode. He refers to a moral certainty that was appealing to him as a youth, but as he grew older he realized that it was far more complicated than that." 
In the The Sky's the Limit short story "Turncoats", which takes place shortly after his defection, DeSeve redeems himself by retrieving and de-activating a Romulan radiation grenade that has been placed aboard the Enterprise-D, an action which ultimately leads to his death. Picard recommends him for a posthumous commendation and requests a funeral with full military honors for the ensign.
DeSeve also has a card in the Star Trek Customizable Card Game.