Ilya Koslov is a character on The Blacklist and an old friend of Raymond Reddington.
Ilya Koslov is a former KGB intelligence officer and a childhood friend/lover of Katarina Rostova. Also, Ilya is one of Raymond Reddington´s oldest and dearest friends and they seem to know each other since they were children. After Katarina Rostova staged her suicide in Cape May, Ilya helped her disappear. Ilya devised the plan to impersonate Raymond Reddington in order to gain access to the money the Cabal placed in his name to frame him for treason, so they could fund Katarina´s new life. A year later, Ilya was called by Katarina Rostova´s father Dominic Wilkinson to a meeting at a cafe in Belgrade. Dom told Ilya that his plan with Katarina didn´t work and informed him that Katarina´s enemies joined forces and formed the Townsend Directive to hunt and kill her. Afraid that the Townsend Directive could harm his granddaughter Masha, Dom enlisted Ilya to help him kill Katarina to calm down the Directive and keep Masha safe. Still in Belgrade, Ilya, following Dom´s plan, lured Katarina to an ambush where he and Dom tried to kill Katarina with a car bomb in front of the whole Directive, but they failed and ended up killing her new husband instead. However, this was not the real Katarina but Tatiana Petrova, an asset whom Ilya had worked with and whom Dom and Ilya had set up to truly fake Katarina's death. After that, Ilya went into hiding. He started a family and left the life of espionage and crime behind him, though he still kept in touch with Raymond Reddington.
Ilya Koslov first appears after Katarina sends him a coded message through an online crossword puzzle, which he receives and decodes at his office desk. He follows the clue to her hideout at a local 2 story motel. He is elated to find her alive, even though she is not happy or all that well. She is desperate for help to escape Anton Velov and his insatiable quest to capture her. He assists her in sneaking back into Moscow to find her father. When her attempt to sway her father results in their near capture/kill by Velov’s men, Ilya slips in unseen and kills the squad. Once Katarina says her goodbyes to Dom, Ilya and Katarina return to hiding at a Moscow hotel. Ilya finds Katarina standing on the edge of the terrace balcony, ready to end it all to stop the hunt and protect her daughter. Ilya then suggests there is a better way, that instead of falling on their swords, they can pull off a “bold plan” to take over the late Raymond Reddington’s assets by stepping into his shoes to show up at the banks. Dom concludes his retelling of this story to Liz with the answer that Ilya Koslov is the man who became Reddington nearly 30 years ago.
Ilya meets Red at a waterside park bench. They reminisce over childhood and Ilya confirms to Red that photos from Paris confirm the location of who they’ve both been looking for. He asks Red if he’s sure he wants to go through with finding her.
It is revealed what happened in Belgrade.
As Liz learns the truth about her past, Ilya appears both as an image talking to her and in flashbacks. When Katarina confronts the real Raymond Reddington in an attempt to get her daughter and the Fulcrum back, Ilya is part of a team that joins her and after a young Liz shoots her father, Ilya takes her from the house.
It's revealed that Dom had told Liz most of the truth about the story surrounding Katarina and Ilya following Katarina's suicide attempt, including that Ilya had posed as Reddington and withdrew the money that Katarina had used to frame the man. However, Ilya reveals that Dom had lied about Ilya being the one to assume Reddington's identity full time. He also reveals that the attempt that he and Dom had made on Katarina's life was actually not on her but on an asset named Tatiana Petrova whom Ilya had been working with. Ilya was left guilt ridden when their failed attempt to assassinate Tatiana resulted in the death of her husband instead, but he and Dom had successfully faked Katarina's death while Tatiana was later the one to chase after Reddington and to torture Ilya for answers, not Katarina. Ilya helps Katarina explain to Liz that Katarina had created the fake Raymond Reddington and his syndicate in order to protect her daughter from her many enemies.