Constantin Rostov is a criminal and a member of The Blacklist.
Rostov is a former KGB agent and the stepfather of Elizabeth Keen, who he truly believed to be his biological child.
After the fall of the Soviet Union, Rostov, adopting the alias "Alexander Kirk", took advantage of the broken economy to purchase several mining and oil operations at low cost. In 2002, following a series of politically motivated raids on his companies by the Russian government, Rostov fled the country.
He is now an oligarch, who not only is a billionaire in the energy field among his many other criminal enterprises, but also has his connections to many political powers across the globe, financing their rise to power, including current presidential hopeful Senator Robert Diaz. He and Red have a long personal history. They appear to go back 20+ years to the fall of the Berlin Wall and the end of the Cold War. This appears to be an allusion to the "war" Red has been mentioning.
Rostov has a rare blood disorder (A trait that has been passed down from father to son in the Rostov family), which requires frequent transfusions. Even so, it is apparent that he has less than a year to live. A possible cure for Rostov is to use STEM cells from either Liz, or Agnes, as they are both immune to the blood disorder. Red explains to Liz that this is the reason of Rostov's obsession of her and Agnes.
When Reddington interrogates Susan Hargrave, she tells him that Halcyon Aegis was contracted by Alexander Kirk to find and deliver Elizabeth Keen.
It is confirmed with a DNA report which can be questioned in its authenticity in Alexander Kirk's SVR file that he is Elizabeth Keen's father.
After being captured by the FBI, a DNA test revealed that Constantin Rostov is not in fact Elizabeth Keen's father. Liz confronts Rostov about this, who refuses to believe it.
Rostov escapes federal custody and kidnaps Elizabeth, making his personal doctor run the DNA test again, only to get the same results. His lawyer and secret lover, Odette, calms him to believe it. Red calls Rostov to make a deal: his life for Liz's own. Once Red arrives, Rostov tells Odette to let Liz go. Once alone, he has his men tie Red strapped to a chair and revealed poisons underneath it. Despite Red's offer to help cure him, Rostov refused and demanded answers from Red in whether he is Masha's father. While poisoning Red, a few of Red's men arrived with a former patient of Sonia Bloom's in order for him to prove to Rostov of Bloom's cure. After ordering their men out, Rostov demanded Red to tell him the truth. Under poisoning, Red finally confesses to Rostov that Liz is his daughter. As he is about to kill him, Red asks him on how he and Katarina met, wanting to remember what kind of woman she was. Rostov answers that she dared him to enter a house filled with people, calling it a moment he saw the true Katarina. Before Rostov kills him, Red whispers something into his ear, making him stop. The 2 stare a look before going their separate ways, leaving Rostov's whereabouts and fate unknown.
When Liz questions Red about Rostov, he only responds by telling her that he won't be bothering them anymore.
Red is confronted by Harold Cooper for let Rostov go. All that Red will say is that Rostov is "gone" and that he won't be coming back before changing the subject.
- He addresses Elizabeth Keen by her Russian birth name, Masha Rostova.