Marcel first crossed paths with NCIS in a sting operation they arranged back in August 2011 when they thought his name was Zahid DeGramont and was the leader of a French-Morrocan arms-dealing gang. He kills someone in L.A. and Callen and his team are assigned the case. They arrange a drop-off where all Zahid's hired guns are killed in the takedown. He tries to escape but is shot in the face and taken into custody.
While he was in the recovery ward for his injury he convinces everyone that he was just the driver of the group named Karim Ghilas, when in fact Ghilas was the one pretending to be DeGramont. He talks to Callen where he reveals the location of the guns he was going to sell and successfully fools Callen into thinking he was Ghilas. He pleads out and is sentenced to six years in prison. One month later he was scheduled to leave the prison ward to serve his sentence. Before that could happen he paid an inmate named Hector Ramirez's family $100,000, whose sentence was over, to take his place and paid a prison administrator named Wendari to arrange the switch.
After he escaped he continued to act as the middle man for various deals and later kills more people in Russia and Brazil using the same M.O. and keeping his name out of it. He comes back to L.A. where he kills two pharmaceutical reps when their boss was going to back out of selling some expired meds. Later an Interpol agent named Martin Kallstrom was assigned the case and tracked DeGramont down to L.A. where DeGramont killed him in his hotel room and hired a man recently released from a Swedish prison to impersonate him.
Three days later he acquires the meds and sends his Kallstrom impersonator to infiltrate NCIS. He then toys with NCIS and pretends to kill his impostor and later arranges a deal with the Nigerian diplomats who hired him and sells the medication. Afterwards he poisons his accomplice and calls Callen to taunt him. He makes his escape and vows to get revenge on Callen for shooting him in the face.
Marcel later uses his old contacts to murder an undercover NCIS agent named Mike Renko. Unfortunately he overestimates their intelligence and they instead kill one of their own people. He then kills the killer (Earl Mayfield) and later targets their local sniper (Walter Henderson) in a temporary attempt to frame him for the murders. After that he targets the undercover agent and shoots him, where he dies from his wounds in the hospital. After it becomes apparent to NCIS who the real killer is NCIS try to track him down and eventually find him, only to see him kill another NCIS agent named Lauren Hunter with a car bomb.
He is taken into custody where he is confident he will be let go due to his capture of a important analyst. NCIS try to stop the deal, discovering more bodies and bobby traps along the way, but ultimately fail and the deal for Marcel's freedom is set up. Only they realize he already sold the information the analyst had to an enemy country but it's too late to stop the deal. Marcel tries to secure his freedom with several precautions like TV cameras and local cops everywhere. He underestimates Callen's cool and in revenge for killing Mike Renko and Lauren Hunter, an enraged Callen guns him down in revenge, resulting in Callen being arrested for his murder.
But it turns out that NCIS had faked Marcel's death by putting a bulletproof vest on him and was being held hostage by NCIS, who later arranged a trade between agent Naseem Vaziri for G. Callen, which was successful and he was taken to Iran for questioning for his crimes in Iran.
He reappeared in the Season 4 finale when he was held hostage by Naseem Vaziri's men and it was discovered that his left hand was missing. NCIS broke him free in order to arrange a deal between the Iranians and Sidorov's men regarding the nuclear bombs. When he conducted the deal he covertly informed Sidorov of David's status as an undercover agent and later taunts Callen with this and Hanna and Deeks are tortured by Sidorov. After he taunts Callen more, NCIS lets him go free in order to get Sidorov to release Sam, and Deeks, however he escapes by killing a custodian and ripping off his tracker with his own skin
He then lures Vaziri into a trap by having someone fake being his daughter Adeline, and then murders her and cuts off both of Vaziri's hands to payback Vaziri cutting off his hand, he then lets him lives to suffer in pain and shows NCIS. He then proceeds to escape in a car, but is injured and ultimately attempts to get Callen to kill him, but Callen refuses and puts him in jail with a triple sentence. Janvier boasts that he will escape and get revenge, referencing the book, The Count of Monte Cristo, but Callen doesn't believe it and leaves him in prison.
It is revealed that Janvier still living in prison with a Christmas card sent to Callen in "Cancel Christmas".
"My dear Callen from prison, with love. Happy holidays... Thinking of you, Janvier."
When Marcel is conducting business transactions his M.O. is to buy the supplies for his employers and then arrange a drop-off for the merchandise. He usually hires a group of guns-for-hire to take care of the deal, while he sits in the car posing as the driver giving the orders. When caught he arranges for his hired guns to take his identity just in case they are caught.
Marcel's murders all took place around different continents. His victims were people who worked for the seller of items he was interested in when the seller tries to back out. He shoots his victims in the legs to incapacitate them and then has them call their boss. After this he would tell his victims to tell their boss that if they tried to back out of the deal he would kill the seller's family and loved ones. While they did this he would pour gasoline on his victims in preparation for his next move. He then has them hang up the phone and sets his victims on fire, effectively killing them by burning them alive.
When he came back to L.A. he changed his M.O. to shooting people from a long range. He specifically targeted the jaw, symbolizing the moment Callen shot him in the jaw. He also planted bombs specifically targeted NCIS agents working with Callen. Callen then makes him go out in the open to get Sidorov to find them, however he murders a Custodian and takes his clothes and cuts off his tracker along with part of his skin.
He then lures Vaziri into a trap by having someone fake being his daughter, and cuts off both of Vaziri's hands to payback Vaziri cutting off his hand, he then lets him lives to suffer in pain and show NCIS. He then proceeds to escape in a car, but is injured and ultimately attempts to get Callen to kill him, but Callen refuses and puts him in jail with a triple sentence, boasting that he will escape and get revenge, referencing the Book The Count of Monte Cristo, but Callen doesn't believe it and leaves him in prison
- An unnamed person in L.A.
- An unnamed man in Russia
- An unnamed man in Brazil
- Paul and an unnamed pharmaceutical rep in L.A.
- Interpol Special Agent Martin Kallstrom
- Fake Martin Kallstrom
- Earl Mayfield
- Mike Renko
- Walter Henderson
- Lauren Hunter
- The warehouse bombing (no deaths):
- Six unnamed guards
- Unnamed custodian
- Kayla Bishop
- Two unnamed guards
- Naseem Vaziri (cut off both his hands)
- G. Callen (attempted to run-over)
Marcel Janvier is a remorseless sociopath incapable of feeling or even expressing any kind of remorse or regret for the pain he caused along with the crimes he committed. He had no moral compass and as such, killed whoever he felt like killing. However, Janvier doesn't have any regard for his own life (he barely flinched when Callen tried to shoot him twice, and did not hesitate to cut off a layer of his own skin) Janvier is a chess player as shown in Episode:Sans Voir, Part 2 and a talented one: he sets the board as he wants it and then lures his opponent into a deadly trap by being able to think three moves ahead of what they are going to do. He was able to beat the entire NCIS team in his deadly mind game without missing a single one of his moves.
Janvier is also a ruthlessly effective killer. Although he seems to know only English, French and Romani as languages, he can imitate nearly every accent which he uses to great effect during his killings and he can disguise himself almost flawlessly whether it be as someone else or as a person he is not. This is what earned him the nickname 'The Chameleon'. He is so good that, even G Callen, himself one of the most skilled undercover agents in America, admitted that Janvier was better than him. He is also skilled at spy-craft being able to capture NSA analyst Kelvin Atlee and NCIS Special Agent Lauren Hunter with ease.
However, Janvier is not incapable of error: this was proven the day he was shot by Callen. He was unable to predict that Callen and Sam were federal agents working undercover and this mistake cost him by the way of a bullet to the jaw and, almost his freedom.