Isaak Sirko (also known as Volk, Russian for "The Wolf") was a character on DEXTER. Isaak was the leader of the Koshka Brotherhood, which based their operation in Kiev, Ukraine, where Isaak lived. He was the owner of a chain of gentlemen's clubs known as The Foxhole.
He was the Main Antagonist of Season Seven and the Seventh Main Antagonist in the series. He is the second antagonist to be from a foreign country (the first one being Lila West). Isaak possessed excellent fighting skills, as demonstrated when he killed three armed men in a confined space, despite having been lured into a trap by Dexter Morgan. He was also highly influential, able to organize the loss of evidence leading to his release from prison. Isaak is the third major antagonist to have been killed by someone other than Dexter Morgan (the first two being James Doakes and Jordan Chase, and the fourth being Maria LaGuerta).
In Season Seven, Isaak comes to Miami to investigate the disappearance of one of his men, Viktor Baskov, whom he put in charge of operations there (and who is later revealed to have been Isaak's secret lover). He soon discovers that Viktor was murdered by Dexter Morgan, and becomes determined to kill Dexter simply for revenge.
Isaak is a tall, handsome, Caucasian man in his late forties with graying hair. His attire consists mainly of expensive suits that were most likely imported from Ukraine or Russia, and is always very professional-looking.
Isaak is a lethally dangerous yet sophisticated man with a calm demeanor. He is one of the leaders of the Koshka Brotherhood, and is respected and feared among his subordinates. Despite his status as one of their highest-ranking officials, Isaak has no qualms about getting his own hands dirty, and will usually opt to perform a murder himself rather than having one of his men do it. An example of this is when he murdered Tony Rush by stabbing him in the eye with a screwdriver, despite being accompanied by bodyguard Jurg Yeliashkevych.
Unlike most Dexter antagonists, who are portrayed as entirely self-centered, Isaak possesses a strong sense of personal honor, and appears to hold at least some regard for those who work under him. A prime example of this is when he learned of Viktor's death. Isaak became enraged and determined to avenge his former associate, and would not allow anyone to speak ill of him (it should be noted that he and Viktor were lovers, and whether he would go to such lengths for any other of his men is doubtful). His desire to avenge Viktor was so strong that it ultimately led to him alienating the other Koshka bosses (who felt it was bad for business), which in turn led to two attempts on his life.
However, Isaak can also be cruel to his men, as shown when he forced Alex Dubrozny to confess to Mike Anderson's murder and then kill himself, so that the police would stop investigating Anderson's death. Isaak does this by implicitly threatening that Dubrozny's family will be killed if he refuses. However, he also promises to send them a substantial amount of money if Dubrozny obeys, and since Isaak is shown to be a man of his word (proven when he keeps a promise to let Dexter live, despite his desire for revenge), it can be assumed that he kept it in this case.
Isaak appears to be a rather easygoing individual and will often talk to those around him in a lighthearted manner, regardless of the situation. He has a clear sense of humor, as shown when, for example, he first speaks to Dexter on the phone. He recalls that Louis called Dexter a "douchebag" with a small laugh. A more twisted example was when Dexter describes how the assassin hired to kill Isaak died in Viktor's apartment: "My my, you couldn't have described it better yourself if you had been there." This is somewhat sarcastic given that Isaak had already deduced that it was Dexter who killed the assassin. He may also have a more forgiving nature than any other antagonist, as despite his grief over Viktor's death, he ultimately abandons his quest for revenge against Dexter (after Dexter saves his life by helping him kill two more assassins).
Isaak is also portrayed as romantic, admitting that his desire to kill Dexter is entirely fueled by the love he had for Viktor. He believes love to be something that transcends reason, citing his relationship with Viktor as an example (since he and Viktor were not only very different people—Isaak being calm and professional, Viktor being reckless and impulsive—but both men would have been kicked out of the Brotherhood and possibly killed if their homosexuality was found out). He is baffled by Dexter's apparent cluelessness when it comes to matters of a romantic or personal nature, even going so far as to give him advice—"She needs to know how you feel"—on how to maintain his relationship with Hannah McKay. The ultimate proof of his romanticism comes after he is shot by George Novikov. Realizing that the wound is fatal, Isaak's final wish is to be buried with Viktor.
Early life and family history
Isaak was born on May 25, 1964, sometime after the death of Nadezhda Obukhova, in Kiev, Ukraine. In his childhood, he pushed his music teacher, Mr. Chillich, down a flight of stairs for giving him an F, remarking: “Isaak, you think you’re strong, but you are not strong if you don’t feel.” Isaak's father then sent him to a disciplinary school in England, which explains his British accent. As a result of his time at the school, Isaak claims that his love of opera is all that remains of his softer side.
Before his birth, Isaak had a great-uncle who served in the Red Army and was loved and cherished by his people in Kiev. A Soviet Party Official became jealous of his fame and attempted to make an example of him by taking away his house. After protesting, Isaak's great-uncle was executed by hanging. Isaak's grandfather (who was just a teenager at the time) developed an intense desire for revenge, which the official saw in his eyes. The official had him arrested and sent to Siberia, until the Soviet Union fell and Isaak's grandfather was released to celebrate with all the others in Kiev. It was there that he found the party official and, after waiting many years, killed him by stabbing him in the kidney with a six-inch knife.
Viktor kills a Cop
Detective Mike Anderson is driving home when he spots a car parked alongside the road. He steps out and sees Viktor Baskov, one of Isaak's closest subordinates and part of the Koshka Brotherhood, attempting to fix it. Mike offers to help and, despite Viktor declining, opens the trunk and discovers the body of Kaja Soroka. Viktor shoots Mike and wipes down the car and weapon. He also takes a bracelet (which contains a GPS chip designed to keep track of the Brotherhood's mules) from Kaja's body. Viktor then drives off in Mike's car, leaving him dead on the concrete, and contacts someone within the Koshka Brotherhood (possibly Isaak himself), notifying them of his mistake of killing a cop. He arranges a flight to leave Miami immediately for his own safety and return to Kiev, Ukraine.
The next day, Dexter Morgan arrives at the crime scene and discovers the shooter's identity from a fingerprint that Viktor forgot to wipe off his turn signal lever. He keeps this information to himself, seeking a new victim to kill, and eventually captures Viktor at the airport before the latter can leave Miami. During Dexter's ritual, not much is learned as to who Viktor is or why he did what he did, but Viktor asks if someone hired Dexter to kill him. Dexter confirms it's just his own doing, and Viktor asks if there's any way that he can convince Dexter not to do this. Dexter replies with a no and Viktor says to "get it over with." Dexter then wraps Viktor's head in plastic and uses a nearby fire extinguisher to smash Viktor's skull in.
As Miami Metro Homicide seeks answers as to who killed Mike, they discover that Kaja worked at a night club named the Foxhole, which is run by George Novikov. They stop by and inquire as to who could have killed Kaja and Mike, but George claims not to know anything about it. After beating around the bush a bit, George finally gets some peace and puts a call in to his boss Isaak Sirko.
Isaak is at his home in Kiev, Ukraine when the call comes in to his associate Jurg Yeliashkevych. Isaak takes the call, and George informs him that Viktor made his flight and should have arrived in Kiev at 5:25 (Isaak's time). Isaak says "Good" and asks about the dead police officer. George confirms that it's still a problem but he'll take care of it, with Isaak once again saying "good." He hangs up the phone and hands it to Jurg, then gives a private smile, believing that Viktor is safely back in Kiev.
Arrival in Miami
Tensions continue to rise between George and the Miami Metro Police Department, who start shutting down the Foxhole twice a day to seek answers about Mike's shooting. George grows frustrated as detectives question the workers there. One of the Foxhole's strippers, Nadia, gives the police a potential lead when she tells Detective Joey Quinn about Tony Rush, a bouncer who was dating Kaja. They question Rush, who eventually reveals that Kaja consorted with Viktor. Sergeant Angel Batista, along with Quinn and the officers from the previous shutdowns, arrive at the Foxhole and ask about Viktor, which surprises George as he has no idea how they got his name.
When they later head out, George retreats to his office and becomes surprised to find Isaak sitting there. George casually says "Isaak" before correcting himself to the more honorable "Mr. Sirko," and asks what he's doing here. Isaak reveals that he's here to find Viktor. George states he has no idea how they found out his name, which Isaak says he's "sure" about but that they'll get to that later.
Jurg then walks in, closing the door behind him and standing sternly at the door, which makes George slightly nervous. Isaak states that he checked and learned that Viktor never made his flight, and with a hint of suspicion in his voice, he interrogates George.
Isaak: I can't help but wonder...if someone in the Brotherhood decided to make an example of him.
George: Without your approval? No. No-no, Viktor was impulsive, yes. He did a stupid thing, yes. But he was one of us and we would always protect him.
*Isaak puts his hand to George's cheek and leans in with a serious expression*
Isaak: You're in luck.
Isaak: I believe you.
As George sighs in relief, Isaak begins looking into Viktor's disappearance. After learning of Kaja and her connection to Tony Rush, Isaak determines that Rush leaked Viktor's name while being questioned by the police. Isaak suspects that Rush may have had a hand in Viktor's death, considering that he loved Kaja and may have killed him out of revenge.
Isaak, Tony and a Screwdriver
Isaak and Jurg arrive at Rush's house, walking into the garage as Rush works on his motorcycle. Isaak tells a confused Rush that he's his employer (or rather, former employer, as Rush recently gave notice), explaining that he owns the Foxhole and other various enterprises. Isaak jokingly claims to have come to give Rush's exit interview, but then seriously questions him about his reason for quitting his job, which may involve Kaja's death. Rush becomes more and more distraught with emotion as he continues talking, nervously looking between Jurg and Isaak.
When Rush claims that the police never brought him in, Isaak says he had Rush followed. Rush then admits it but denies that he told the police anything. Isaak reveals he was bluffing; Rush was just the most likely candidate. Tony continues to get more emotional and nervous as Isaak comes closer.
Isaak: You loved her, didn't you?
Isaak: Kaja. You were in love with her. It's nothing to be ashamed of. It's the one thing that gives meaning in this life...I would have done the same thing if I was in your position. It's terrible...to lose someone you love.
With Rush's defenses weakened, almost in tears thinking about Kaja, Isaak grabs a nearby screwdriver off the workstation and quickly stabs it directly into Tony's right eye. He steps back as Tony twitches and shudders before falling dead. Isaak calmly looks at his body, taking a moment to examine his work, then leaves the garage. Jurg wipes down the screwdriver's handle before following Isaak out.
"Who is Dexter Morgan?"
Isaak later speaks with Nadia about her relationship with Quinn, and tells her to get close to him to learn more about Viktor's disappearance. He tells George about the GPS bracelet, which Viktor probably took with him so the police couldn't identify it. He asks Jurg to find the man who designed it, after which they pinpoint the bracelet's location: eight miles out to sea. Isaak becomes enraged, realizing that Viktor was killed and dumped in the ocean like garbage. He has the designer find the marina from which Viktor was dropped. While Isaak waits, Batista returns to speak with George, accompanied by Dexter. Isaak unknowingly has a casual conversation with the man who killed Viktor, telling him to relax since he looks tense. Dexter reveals that he's not here for women or alcohol, and Isaak jokes that Dexter's in the wrong place. When Dexter takes notice of Isaak being from another country and asks what he's doing here, Isaak says he's looking for something, something Dexter can agree with. Isaak notes they're both here to kill time and remarks on how ironic it is that they're here for a different reason and yet the net result is the same: they're both frustrated. Dexter agrees, and wishes Isaak luck in finding what he's looking for, not knowing that Isaak is looking for him.
After locating the marina, Isaak, Jurg, and George head there and find the boat, the Slice of Life. Louis Greene is trying to sink the boat with a drill, but Isaak stops him and asks if he's the boat's owner. Not wanting to get in trouble, Louis affirms this, and Isaak angrily asks about Viktor. As George restrains Louis, Isaak takes the drill and threatens to put Louis' eyes out. Louis caves and tells the truth: he reveals Dexter's name and occupation, and explains that he was trying to sink the boat because of a grudge against him. Isaak then shoots Louis in the head and leaves George to dispose of the corpse. Back at the Foxhole, Isaak examines both Dexter's and Louis' files, and notes their connection to the police department; he deduces Viktor's death was revenge for killing Mike. George insults Viktor's brash recklessness, but Isaak defends him, stating that he was a good man and refusing to let anyone speak ill. When George questions why the police are still coming by, Isaak knows this must be off-the-radar, that Dexter killed Viktor without anyone else knowing. Isaak decides to have someone take the fall for Mike's murder, and decides on bartender Alex Dubrozny. Isaak, George, and Jurg visit Alex's house at night. Isaak coerces him to write a confession note and then commit suicide so that the police can wrap up their investigation and take their eyes off of the Brotherhood. Isaak promises to send Alex's family money, and Alex reluctantly goes through with it. The police arrive the next day and conclude that Alex was the killer, though Batista isn't convinced and Dexter knows Viktor was the real perpetrator. Isaak later goes to Viktor's apartment and finds a hidden photo of him and Viktor in Kiev. Crying, Isaak swears to Viktor that he will avenge him.
The Hunt for Dexter and Imprisonment
As Dexter is about to enter his apartment, he notices his curtains are open slightly and realizes someone might be inside. Isaak sits in the apartment with his gun and a set of kill tools, but Dexter leaves a message on his machine, posing as somebody else and confirming a meeting that's about to take place soon. Isaak then leaves disappointed, but Dexter watches from afar and recognizes Isaak from the club. He determines that the Koshka Brotherhood found out he killed Viktor and sent a hitman after him. Isaak goes to a restaurant for a small coffee and gets a call from Dexter asking why he was in his apartment, prompting Isaak to react in amusement. He answers that he wanted to have a "chat" with him about Viktor, and brings up the incident with Louis on Dexter's boat. Isaak says that Louis gave him Dexter's name and referred to Dexter as a "douchebag." When asked if a boss paid Isaak to do this, Isaak denies it; Viktor was a friend of his, so this is personal. When Dexter brings up that killing Viktor settled the score for him killing an innocent woman and a cop, Isaak says he's not stopping with Dexter—he wants all of Dexter's cop friends involved in this vendetta, including Debra Morgan. Dexter yells that only he killed Viktor and no one else was involved. Isaak then spots Dexter watching him and says "Hello, handsome." Dexter hangs up and leaves.
When Dexter wraps up at a crime scene with Hannah McKay, Isaak begins following him. Dexter catches on and drives to a Colombian bar, where he goes inside and says "Koshkas" to let them know of Isaak; he soon sneaks out through the bathroom window. Isaak comes in with his gun and, upon seeing the three Colombians, smiles in amusement as he realizes that Dexter set him up. Isaak successfully kills all three Colombians while only getting a minor bullet graze wound on his head and a knife wound on his left forearm. Back at the Foxhole, he has his arm patched up by Jurg. When Joey Quinn comes to see Nadia, Isaak learns that Quinn used to work for the Koshkas as a dirty cop; he has Jurg put money in Quinn's car, enticing him to become their mole. Thanks to blood evidence at the crime scene, Isaak is later arrested for killing the Colombians, but he stays calm and confident that he'll get out.
While in jail, Isaak receives a visit from Dexter, who wishes to know why he wanted to kill him so badly. Isaak laconically answers, "You killed Viktor. I liked Viktor." He then questions Dexter's motive for killing Viktor, and Dexter says that it was revenge for Mike, but Isaak notices a brief hesitation in his voice and knows he's lying. Dexter asks Isaak if this is now over, but Isaak warns him that it isn't and that they will see each other again. Later, Isaak is accosted by a fellow prisoner—a Colombian gang leader—who threatens him for killing three of their men. But Isaak easily incapacitates him with an arm lock and says even if he kills him, the Koshkas will easily retaliate. Breaking the leader's hand, Isaak takes his cell phone and calls George to visit him, wanting an update on Quinn. George arrives and reveals that Quinn has returned the money, but Isaak tells George to keep trying.
George later confronts Quinn and threatens to deport Nadia to much harsher working conditions in Dubai. Quinn gives in, and George tells him to take the evidence against Isaak out of Miami Metro's lockup. Reluctantly, Quinn goes into evidence and steals the blood samples.
An Early Release and Resuming the Hunt
Due to the evidence going missing, Isaak is released from prison and greeted by Jurg and George, but Debra has ordered police officers to tail Isaak 24/7. George suggests that Isaak leave Miami, but Isaak refuses until he's killed Dexter. He pays Dexter an unannounced visit as the latter dines at an outdoor restaurant. Unable to kill him here thanks to the police watching, Isaak sits down to talk and get to know Dexter better. He assures Dexter that he won't be lured into a trap again, but Dexter compares him to the cartoon character Wile E. Coyote, who never catches the Roadrunner and just "rocket-skates right off the cliff each time." Isaak chuckles and states that he has motivation for wanting Dexter dead, but he still doesn't understand what drove Dexter to kill Viktor. It couldn't have been revenge, since Dexter only knew Mike Anderson for less than three months.
Isaak: No. You didn't kill Viktor out of vengeance. I have a feeling that you're a different kind of animal. The question is: what kind are you, Dexter Morgan?
Dexter: The kind who hunted your friend down and strapped him to a board and put a plastic bag on his head and...crushed his skull with a fire extinguisher. The kind who's gonna do the same thing to you, give or take the fire extinguisher.
*Isaak angrily composes himself*
Isaak: You are gonna regret sharing that with me.
Isaak later gets rid of his police tail by having them follow a decoy, giving himself enough time to drive out and go after Dexter. He finds Dexter at a donut shop and tries to shoot him through the window, but due to a stroke of luck, Dexter manages to avoid the bullet fire. Isaak drives away, planning to try again later. In the meantime, George fears that Isaak's obsession for revenge might explode in the face of the Koshka Brotherhood, and calls Kiev. With the Brotherhood's approval, George hires an assassin to kill Isaak in his own apartment. However, this fails as Dexter, also attempting to kill Isaak in his home, encounters the assassin and is forced to kill him by slitting his carotid artery. The Miami Homicide Department, including Debra, Masuka, Batista, and Dexter himself, investigates the murder while Isaak is present. Since the police were tailing him at the estimated time of death, Isaak has an alibi. When Dexter recreates the murder sequence, Isaak jokingly remarks that Dexter couldn't have described it better if he had done it himself, insinuating that he knows Dexter is responsible. Dexter calmly replies, "Thanks," and leaves the investigation to Masuka.
Later, Dexter follows Isaak to a bar, planning to sneak in, inject Isaak with M99, and carry him out as if he's helping a drunk friend to his car, misleading the police escorts. Inside the bar, Dexter approaches Isaak and remarks that he cannot kill him here. Isaak calmly replies that he can but he won't, and orders Dexter a beer so they can both relax. Dexter soon notices they are in a gay bar, and deduces that Isaak came here so that his enemies wouldn't follow him inside, but Isaak replies that he simply feels comfortable here, revealing that he is homosexual. Dexter tells Isaak to cut his losses and go home to Kiev, to which Isaak replies that there is nothing for him in Kiev anymore, since Viktor is dead and the Brotherhood has turned against him. Isaak then reveals that he and Viktor were lovers. Dexter states that neither of them has to do what they do, but Isaak insists that he does, and asks what Dexter did to Trinity for killing Rita. Dexter says it's nothing he wouldn't do again, but it didn't bring Rita back, just like killing him will not bring Viktor back. When Dexter asks what Isaak will do after he kills him, Isaak replies that he will look for a nice corner of the world to wile away his days in comfort and anonymity. Isaak says that they are both outsiders, watching the people around them, pretending to be like them but knowing they are not—the best they can hope for is to find a place where they don't have to pretend. Isaak laments the fact that he and Dexter could have been great friends under different circumstances, and leaves money to pay for their drinks. As Isaak leaves, Dexter can't bring himself to go through with his plan. Later, Isaak sorrowfully listens to a voice message left by Viktor, saying he will arrive soon in Kiev.
Teaming up with Dexter and a Final Resting Place
Isaak and Jurg later learn that George has hired two more assassins: Benjamin Caffrey and Oleg Mickic. Isaak has done business with these two men in the past, so he calls and tries to persuade them to drop the hunt. When they refuse, Isaak finds himself cornered and decides to seek help from Dexter. At Dexter's apartment, Isaak promises to spare his life in return for his assistance in killing the two hitmen. Dexter declines, but Isaak later abducts Hannah McKay, who has become Dexter's girlfriend, and has Jurg hold her hostage. While Dexter is out, Isaak breaks into the apartment and makes some tea. When Dexter returns and learns what Isaak has done, he angrily tries to pin him to a wall with a knife, but relents for Hannah's sake. He insists he's the wrong man for this, but Isaak knows better, since Dexter expertly killed Viktor and the hitman in Isaak's former apartment. Dexter reluctantly agrees to help.After discreetly stabbing Mickic dead at a shooting range, Dexter visits Isaak's hotel room and asks to speak with Hannah via Isaak's cell phone. While speaking with Hannah, Dexter snaps a picture of Hannah in her holding room and sends it to himself. When Hannah says, "I miss you," Dexter merely replies, "Likewise," which annoys Isaak. After the call is over, Isaak chastises Dexter for his lack of romanticism. This segues into a heated argument over Viktor, in which Isaak holds a gun to Dexter's head. Dexter pushes Isaak, saying that he himself is responsible for Viktor's death by sending him to Miami. Isaak eventually lowers the gun angrily and tells him to leave. While at a crime scene, Dexter notices Caffrey watching him, and calls Isaak. Isaak comes up with a plan where Dexter will lure Caffrey to a docked ship in the harbor, belonging to the Koshka Brotherhood. When Caffrey approaches Dexter on the ship's bow, Isaak shoots and kills him. Dexter and Isaak throw the body in the water, along with their weapons as a sign of trust. Isaak says, "See? Man of my word," and promises to leave Miami and Dexter alone. As Dexter stays on the ship to clean Caffrey's blood, Isaak runs into George, who has followed Caffrey and come to kill Isaak himself. Unarmed, Isaak mockingly tells George to just do it, and George shoots Isaak in the stomach at point-blank range. Instead of finishing Isaak, George flees when Dexter approaches. Dexter offers to take Isaak to a hospital, but the latter knows that nothing can prevent his death and requests a final favour from Dexter: take him to the place where Viktor Baskov was dumped.
Sailing on Dexter's boat, the two men discuss how Dexter is "absolutely fearless in the face of death and yet so afraid to live." Dexter explains that death's predictability and inevitability calms him, but ever since he met Hannah, he feels like he has no control over his life. Isaak replies that he was the same in the past, detached from the world, hiding a secret—it wasn't until he met Viktor that he finally felt alive. With his last breath, Isaak says that there is still hope for Dexter.
Genuinely saddened by Isaak's death and regretting that they didn't become friends, Dexter honors Isaak's final wish and presumably throws his body into the ocean, allowing him to rest in peace and be together with Viktor again.
- My music teacher, Mr. Chillich, he said to me, “Isaak, you think you’re strong, but you are not strong if you don’t feel.” He gave me an “F,” so I pushed him down a flight of stairs. I felt bad about it. After that, my father shipped me off to a disciplinary school in England. Now I’m afraid my love of opera is all that’s left of my sensitive side - Isaak, to Nadia.
- I had a great-uncle, my grandfather's brother. He was a hero, fought for the Red Army during the second World War. When he returned to Kiev afterwards some fat little party official...resented the fact that everyone admired him. So he decided to make an example of him...by taking away his house. When my great-uncle protested, he was hung in front of the counsel chambers. My grandfather was just a teenager...when that party official saw that hatred in his eyes, he had him arrested and sent to Siberia. And the years rolled by...and my grandfather got on with his life the best he could. He was a patient man. Finally, when the Soviet Union fell, he returned to Kiev to celebrate in front of City Hall with all the others. And thats when he found that fat little party official...and he got to enjoy the pleasure that belonged to him and him alone. By sticking a six inch knife into that fat little man's kidney. So you ask when...if this is over. No. I'm afraid not... - Isaak to Dexter, who asked if Isaak's quest for revenge over Viktor's death was over (when he was incarcerated).
- Mr. Chillich - Isaak's music teacher in his childhood, pushed down a flight of stairs. Possibly killed.
- Unnamed Person - Killed prior to Season Seven, was issued an arrest warrant for his murder.
- Tony Rush - Former employee of the Foxhole and connected to Kaja Soroka (who was also connected to Viktor Baskov). He was killed with a screwdriver rammed into his right eye.
- Louis Greene - Former intern for Miami Metro Homicide, a video-game designer and antagonist for Dexter Morgan. He was killed to prevent witnesses after learning of Dexter Morgan. Isaak shot him in the forehead with a silenced pistol.
- Alex Dubrozny - Bartender for the Foxhole. He was coerced by Isaak to shoot himself in the head with the same gun used to kill Detective Mike Anderson, after writing a note implying he killed Kaja Soroka, Mike Anderson, and Tony Rush.
- Three Colombians - Dexter intended to have Isaak killed by the rival Colombians, led by and including Andres Rodriguez. Realizing he had been led into a trap, Isaak quickly shot and killed one after the other. He smashed a bottle over the final victim's head and finished him off with a strong downward thrust of his foot, stomping into his skull.
- Oleg Mickic - An assassin sent after him. Isaak coerced Dexter Morgan to kill him by kidnapping Hannah McKay.
- Benjamin Caffrey - An assassin sent after him. Isaak shot him to death with a silenced pistol after Dexter lured him into the trap.
- Dexter Morgan - Tried to ambush him inside his own apartment, but failed when Dexter realized someone was inside. Later tried again when following Dexter, but was lured into a trap with three Colombians, resulting in him killing all three of them and subsequently going to jail for a short time (he was later acquitted when George Novikov blackmailed Joey Quinn into eliminating the blood evidence linking Isaak to the murders). Later, Isaak tried to shoot Dexter in a drive-by but failed again, as Dexter had accidentally dropped his keys just in time to duck. Isaak eventually decided to allow Dexter to live, so long as Dexter helped get rid of the two hitmen after him.
- Isaak Sirko is the wealthiest and most powerful main antagonist in the series. The only other antagonist that compares is Jordan Chase, who made millions on his motivational self-help books and conventions. Chase even had his own major office building and security force, although he was not an accustomed fighter and, thus, not as openly brutal and lethal as Isaak.
- According to a podcast interview with Michael C. Hall, Isaak was initially meant to be killed by Dexter (presumably by beating him to death with a fire extinguisher like Viktor Baskov) and used as bait to lure LaGuerta in the season finale, but scheduling conflicts forced them to change the script.
- Isaak is the second Main Antagonist to have a loved one killed by Dexter (Viktor Baskov). The first is Miguel Prado, although Miguel does not discover this until Dexter reveals it to him in his final moments.
- Isaak is the fourth main antagonist who's not a serial killer, or at least not known to be one.
- Isaak is also the only main antagonist to be killed by another antagonist and overall the second Main Antagonist to be indirectly killed by Dexter.
- Unlike previous seasons, in which Dexter was trying to catch the antagonists, Isaak is the one trying to track down Dexter. The reason for this is to avenge his lover Viktor Baskov, the killer of Mike Anderson and one of Dexter's more recent victims.
- Isaak has a love of opera, stating that Nadezhda Obukhova is a singer to whom he often listened in his youth.
- Isaak's favorite drinks are small coffees, as he's seen drinking at least one in every episode.
- Isaak is the third main antagonist to have his body thrown into the water (the first two being Arthur Mitchell and Jordan Chase). However, Isaak is the first to not be stripped or chopped up.
- Before causing someone's death, either by his own hand or otherwise, Isaak almost always appears calm. He will become friendly with his victim, smiling and making them lower their defenses before either immediately killing them or proposing something darker.
- Viktor Baskov and Isaak were lovers. In the episode Run, he looked at their photo and wept as he swore revenge. Near the end of Argentina, Isaak reveals that he and Viktor were in an intimate relationship, telling this to Dexter in a gay bar.
- He is known as Vobk (Vovk), which is Ukrainian for "The Wolf". Interestingly enough, his appearance is similar to that of a wolf, with his bright green eyes and darker grey hair. He often wears grey suits and smiles prior to taking his prey...with his expression being fixated, intense and murderous.
- The correct spelling of the word "wolf" in Ukrainian - BOBK (Вовк). The Ukrainian and Russian letters not used "b" - only "B", which reads like a V. Wolf in Russian - "Волк" (Volk). However, the Interpol entry in their database lacks the correct characters in the text and simply spells it "Bobk".
- "Isaak" is a very rare name in Ukraine. On the contrary, Sirko (Сірко) is a frequently used Ukrainian surname. Also, "Sirko" is traditional name for wolf-like dogs and descended from the Ukrainian word for the grey color of dog/wolf fur. "Sirko" is correctly pronounced with the accent on the second syllable.
- Isaak is the second main antagonist in the series to not die in the season finale, the first being Miguel Prado in Season 3. Also, while both of them were once considered "friend" by Dexter, they are almost polar opposites. Miguel begins as an honorable lawman who trusts Dexter almost to a fault, but becomes a hateful murderer and personal enemy of Dexter; Isaak is introduced as Dexter's most powerful personal enemy (and a mobster at that), but the two reach an understanding of sorts in the end.
- Isaak's favorite weapon is a Makarov IJ70-17as with a chrome finishing. Caliber 9x17 (.380ACP).
- Following Season 2, Dexter begins dumping his victims in the Gulf Stream current, with their body parts floating up through the country and becoming difficult to trace to him. For some reason, Dexter chose to dump Viktor in a spot where his body remained in one location, the same location he dumps Isaak's body.
- Under normal circumstances, if Viktor were dumped into the Stream, it would be impossible for Dexter to "reunite" the two, as Viktor would be halfway to Canada. However, it appears that Viktor was omitted from the Stream.