Burke
They say that the worse kind of enemy you can make is a powerful one. Karasov and Hunter made that mistake.
No-one actually knows much about the shadowy figure called "Burke". His rank is unknown as too is his position within Starfleet Command or the Federation, what is known however is that he knew enough people to make life very difficult for Hunter and Karasov. He appeared to enjoy some degree of diplomatic status within the Federation, giving him access to people in high places.
Whilst learning his trade, Karasov trained all of his energies on developing new treatments for Klingon-inflicted injuries such as the agonizer and the mind-sifter. He served on a Klingon medical ship for two years before his posting to the USS Ahrens to find a cure for a Klingon nerve disease. When Starfleet realized that the cure he had found had the potential to be turned into a weapon, they demanded that he stop work on it immediately. Rather than let it fall into anyone's hands, Ilya destroyed the research in front of a Starfleet liaison officer who had demanded it be turned over to Starfleet. Unbeknown to all parties, this officer (Burke) intended to take possession of the information and research with a view to claiming its destruction and then selling it on to the Klingons. Karasov called Burke's bluff by destroying the research. It was for this reason that Burke felt he had a score to settle with Karasov.
Burke initially tried to destroy Hunter and Karasov by putting them on trial for the destruction of USS Pacifica. Burke's influence seemed to be steering the trial away from Karasov and Hunter despite their best efforts. Nathan Hunter in his cell was on the verge of taking his own life when he was interrupted by Karasov who had come across new information. This new information was the survival of some of Pacifica's crew. The crew were rescued and upon the new evidence being put before the trial judge, even Burke could not prevent the trial judge deciding that there was no case to answer. All charges were dropped.
Burke was killed by Nathan Hunter on board USS Farragut following Burke's operatives attempts to sabotage and ultimately destroy USS Iliad in Valnarran space. For more information on this, click here.