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|“||We are brothers, detectives of the NYPD, and strong believers of employing the scientific method in dissecting crime scenes. Due to our modern methods, we are sidelined by our colleagues and overlooked by the police force. But little do they know, our approach will soon be the driving force of our field, and us teaming up with Dr. Kreizler in catching this murderer will prove our technique to be effective.||”|
|— Isaacson brothers|
He's a main character portrayed by Matthew Shear.
"DETECTIVE SGT. LUCIUS ISAACSON is an extremely intelligent, focused man whose medical studies have helped hone his expertise in criminal science. More serious and religious than his fraternal twin brother Marcus, he is nevertheless pleasantly likable with a passion for his work. He believes that bones provide the most accurate information when it comes to forensics. He joins Marcus in helping unravel a disturbing mystery."
Kreizler Institute, 1896. Detective Sergeant Lucius Isaacson presented himself to Dr. Kreizler and John Moore, annoyed by his twin brother Marcus simply addressing him as "his brother Lucius" instead of formally introducing him with the respect his position demanded. When Dr. Kreizler asked for how long he could have enjoyed the benefits of their expertise, Marcus answered all the time he wanted, since he doubted they would have been missed at the police department. Dr. Kreizler, then, speculated if the ostracism suffered at the Police Department's was due to their modern methods of investigation. Marcus nodded, but Lucius added that the fact that they were Jews also influenced that. Sent by Commissioner Roosevelt to assist Dr. Kreizler in his inquiries, both brothers showed themselves skilled and professional in their work, often finding themselves finishing each other's sentences. 
Lucius and Marcus were in a butchery, raging with different types of blades on a cow's severed head, trying to determine what kind of blade had produced the wounds on the body of Benjamin Zweig and allegedly also on that of Giorgio Santorelli. Lucius was frenzied as usual, asking Marcus to let him try too. In response, the brother asked the butcher another knife, ignoring Lucius' requests. When Marcus' attention was directed to a girl delivering flyers on the street in front of the butchery, Lucius hit him on the shoulder with his fist, implicitly suggesting his brother to pay attention to their work. Having found similar knife-marks produced by the last blade tasted on the head's orbital cavities, they bought it.
Coming out of the butcher's shop, Marcus took one of the flyers posted by the girl on a pylon. After taking a look at the content, Lucius sarcastically asked Marcus if he was interested in socialism now. Marcus replied if he knew the difference between capitalism and socialism. When Lucius said he didn't, Marcus explained that "in capitalism, man exploits man. In socialism, it's the other way around."
In the evening, Lucius and Marcus were preparing to leave while Mrs. Isaacson painted fake roses in the dim light of their house. Talking in Yiddish, she asked where they were going since Marcus was using too much scent. The taller twin said they had police business to attend, and Mrs. Isaacson replied that they did not seem to be going to do police business. Lucius stated that their mother intended to say that Marcus "stink like a 10-cent whore", and Marcus, annoyed by his brother's joke, wondered how Lucius could know what smell a prostitute has. While Marcus arranged to turn off most of the candles and lamps, Lucius reminded their mami to turn off the candles before going to sleep. She protested, saying it was Shabbat but Lucius replied that it was not that night.
At Delmonico's, Lucius and Marcus looked around, astonished by such an ambiance, not being accustomed to such a luxury. Lucius prevented one of the valets from taking his doctor's bag. They reached the room booked by Dr. Laszlo Kreizler, where they met Miss Sara Howard. As usual, Marcus introduced himself as Detective Sergeant, introducing Lucius simply as his brother. Lucius corrected him, then asked the woman if he was there to take notes for Commissioner Roosevelt. Shortly after, they were joined by Dr. Kreizler and John Schuyler Moore, taking place around the table. Kreizler proposed a toast to a fruitful partnership, and Lucius told his brother that he did not believe the wine was Kashrut. While the waiters began serving the first course, John Moore asked the reason for the meeting and Marcus stepped in before Kreizler had the opportunity to explain himself. Marcus and Lucius took turns in their explanations about their recent discoveries, showing the diners the weapon bought by the butcher, an Arkansas Toothpick, specifying how such a blade perfectly reflected every prerequisite required by the wounds inflicted on the corpses. They then passed to another discovery, introducing the concept of Dactyloscopy – the science of a finger, palm, or foot leaving a chance impression – and helped by Miss Howard to explain to Moore and Kreizler what it consisted of since she had done some personal reading on the subject. While Marcus showed a magnifying glass on a support containing his and Lucius's thumbs' finger-marks, the latter showed a timepiece found on the burial suit of Benjamin Zweig, bearing a bloody fingermark almost presumably left by the murderer.
At the end of dinner, Sara and the Isaacsons made their way to the exit, chatting about their common interests in the forensic sciences and modern investigative techniques, leaving Kreizler and Moore behind to argue. 
Lucius was arranging some equipment on behalf of Dr. Kreizler with his brother Marcus, while the doctor and Miss Howard questioned John Moore about his whereabouts the night before in Tenderloin. Listening to John's story about the previous night, Lucius said it made sense and agreed with Miss Howard about police involvement to cover wealthy men's sordid visits to the brothels.
Sometime later, the Isaacson brothers and Dr. Kreizler analyzed the fingerprints found on the timepiece projecting it onto a screen using a modern machine. When Dr. Kreizler pointed out a mark, Lucius suggested it could be a partial print but on closer inspection, the three men agreed that it was a scar or other kind of sign on the murderer's finger. Interested in ascertaining whether or not it was a misprint or a real scar, they decided to analyze the Santorelli boy's corpse in search of other finger-marks.
When they reached the morgue, Lucius and Marcus briefly conversed with Mr. Tuthill. In particular, Lucius claimed that Roosevelt was a man thirsty for reform instead of a "bunch of hooey" as stated by Mr. Tuthill. The three men then moved Giorgio Santorelli's coffin on one of the operating tables, only to discover an empty casket.
Late at night, Lucius and his brother were analyzing the body of another child killed on the roof of the old immigration station, soon to be modernized as an aquarium. While Lucius and Marcus were photographing the crime scene, the rest of the team expressed opinions and conjectures about how the murder had taken place. The arrival of Captain Connor and the journalists prompted the team to leave the crime scene, having used more than enough the extra time given them by Roosevelt. 
Appearance and Personality Edit
Detective Sergeant Lucius Isaacson is a man in his early thirties with a slightly tanned complexion, wavy brown hair, and cerulean eyes framed by round glasses. Lucius is a few inches shorter than his twin brother Marcus and a bit sturdier. Although he does not come from high society like Mr. Moore or Dr. Kreizler, he is always impeccable wearing suits, vests, cravats, pocket watch and shiny black shoes.
Lucius is a very intelligent man whose interests range from rising forensic science to psychology and investigative techniques, especially if innovative. His science-minded predisposition does not prevent Lucius to be a very devoted man who lives according to Jewish laws, such as Kashrut. As a Jewish, he has been repeatedly harassed by fellow police officers, but Lucius seems to have developed an armor against prejudice, showing himself to be calmer than his twin brother when provoked. Although he was initially prevented in some respects when it comes to talking about gruesome details of the murders in front of Miss Howard, over time he has become more flexible in his beliefs, coming to recognize and respect the woman's talent as any of his fellow men.
Memorable Quotes Edit
- Mrs. Isaacson: "Vau geystu" (Transl. from Yiddish: Were are you going?)
- Marcus: "We have police business."
- Mrs. Isaacson: "Es tut nisht shmekn vi politsay business" (It doesn't smell like police business.)
- Lucius: "She means you stink like a 10-cent whore..."
- Marcus: "How would you know?"
- Lucius Isaacson: "The rich will always choose dead children over social scandal."
- Lucius Isaacson: "Miss Howard, I am afraid this is no task for the weaker sex."
- Lucius Isaacson (to Marcus): "I came into this world two minutes before you did. I know you better than anyone. I know you better than you know yourself. You're in love with Esther."
- In the novel, Lucius attended medical school before becoming a detective, whilst Marcus trained as a lawyer. Before being recruited by Theodore Roosevelt as Sergeants Detectives for the New York City Police Department, they briefly worked for the Pinkerton Detective Agency.
- In the novel, Lucius and Marcus have a younger sister, Cordelia, named after a character in King Lear, a tragedy by William Shakespeare.
- According to writer and executive producer Hossein Amini, the Isaacson brothers are "Jewish detectives being sidelines by the generally Irish Catholics police force of the time." Director and executive producer Jakob Verbruggen added: "Through the eyes of the Isaacsons, we witness the birth of forensics."
- According to Matthew Shear, the Isaacson brothers are solely interested in employing scientific methods to crime scenes, which wasn't an accepted norm in the 19th century. 
- Matthew Shear and Douglas Smith became close friends while filming the TV series. They built their friendship by researching their characters, watching documentaries together, reading novels and manuals of the time to better understand the mindset of the Isaacson brothers.