Montolo was born as one of two children of Chazz and Rosemary Montolo. In 1971, when he was still a child, Rosemary died of cancer. Afterwards, Montolo moved away to live with relatives in Italy, while his older brother Marco was stabbed to death and the case left unsolved. As for Chazz, he abandoned Montolo and went off to build a criminal empire that operated all across Europe. Eventually deciding to follow in his father's footsteps, Montolo entered the criminal underworld as an adult and became a hitman. He eventually became a member of an online network of hitmen that cropped up after the dismantlement of the Silk Road by the FBI in 2013. At some point during the line of his work, he was hired by a Baltimore woman, Betty Wilson, to kill a drug dealer. Later on, he was hired by another Baltimore citizen, a mechanic named Al Eisenmund, to kill his fiancée.
However, when Montolo kept having tabs on him, Eisenmund panicked, believing Montolo's intentions to be sinister, and ordered his hit as well. On September 23, 2015, Montolo was attacked by another hitman in Atlantic City, New Jersey, but he escaped with a gunshot injury to the jaw that rendered him unable to speak, and he had to rely on a feeding tube in his stomach for his sustenance, which was only supplements. He managed to get the treatment at a local hospital and escaped before the local police officers could arrest him after identifying him. Enraged at this betrayal, Montolo began seeking out former clients of his, killing them in his search for the specific client who ordered his hit. He received assistance and shelter from an ambivalent Betty (who he apparently exonerated and saw her usefulness) and her husband Phil.
After killing Ted Osborn in Albany, New York, Montolo murders William Forrest in an apartment meth lab in Seattle, Washington. Returning to Betty's home, he receives details about his next target, Brian Taylor, from her. The two of them drive to their target's house in Charlottesville, Virginia, and while watching him, he changes his nutritional syringe in order to eat. He then subdues, ties, and gags Taylor, then communicates with him using written notes, asking who betrayed him and showing off his stitches to intimidate him. When Taylor claims to know nothing, Montolo writes that he believes him and promptly kills him, painting a face mask on him afterwards. He then goes after Eisenmund at his auto-body shop. He prepares the syringe, but just as he is about to inject the supplement, he receives a text from Phil and changes plans. He sprays an acrylic on Eisenmund's hands and places a live grenade in them, then slips out the back, taking a ride from Betty and escaping the barricaded scene.
Back at the hideout, Montolo suddenly leaves to Betty's confusion, but comes back to threaten Phil when he arrives home. He forces him in his car at gunpoint and has him drive to a specific location. Getting suspicious after Phil receives a phone call from a person claiming to be Betty, Montolo calls her at home and instead gets the BAU, who had set up a false line. Realizing from the conversation that it is a setup, Montolo tries to kill Phil, but Phil pulls out a concealed handgun and shoots at him. Montolo escapes the car and is chased by Morgan, who he shoots at. Morgan fires back and hits him, causing Montolo to drop his gun and continue fleeing, wounded. Montolo then tries to ambush him, but he is quickly subdued and taken into custody. While in the hospital, he is visited by Morgan, who warns him that the BAU will find the rest of his team, and he gives him the name of his next targets: the Dirty Dozen. Montolo then pulls out one of his stitches to make it look like Morgan tortured him for information, and he smirks at the agent as the latter is forced to leave by a nurse.
Montolo briefly reappeared in the beginning of the episode, where it is revealed he was incarcerated at ADX Florence following his killings. He is led by two guards into an interrogation room, in which he is questioned by Morgan. The agent first asks if Montolo is able to talk, to which he tauntingly replies, "Sí." Morgan then asks about the hitman network. At first, Montolo coughs heavily, then reveals that there are four other hitmen in the network besides him. Morgan tells him to write down their names and addresses. Then, when he asks about the identity of the Dirty Dozen, Montolo just says, "It's not who, but what." When Morgan demands clarification, Montolo starts to cough heavily again and spits out blood before he falls to the ground. Morgan calls for aid and lays Montolo on his side to make sure he doesn't choke to death on his own blood. It doesn't help and Montolo dies. It is later revealed that he was poisoned by a prison guard, who was later found dead from a gunshot wound.
Montolo would be anonymously contacted online by clients who wanted someone killed and be paid $40,000 for each kill. He would receive half of the payment online as a deposit and then the other half once he carried out his contract kills. How exactly he killed his victims during these contract kills is unknown, although Garcia later mentions that he specialized in making his hits look like accidents.
During his crimes in The Job, Montolo targeted his former clients, believing one of them was responsible for hiring the hitman who tried to kill him. He broke into their homes (except for Al Eisenmund, who was attacked in his workplace) silently through a window while wearing gloves, then subdued, restrained, gagged (except for his second client, likely because he was already incapacitated by his recent drug use), and somehow extracted information from them. Once he didn't get the answer he needed, Montolo would then inject the victims in the neck with a syringe filled the same supplement (which consisted of corn syrup solids, soy fiber, and medium-chain triglycerides) that he relied on. This caused them to go into cardiac arrest, guaranteeing a slow death within an hour of the injection. His signature for those crimes was painting a distinctive face mask on the victims' faces with greasepaint as they died, the coloration of which was based on an inversion of the pulcinella (an Italian clown archetype) as a reference to his inability to speak at the time. After the BAU began to close in on him, Montolo began relying on traps and also used a 9mm Beretta 92FS handgun against his later victims.
The unsub is a highly skilled and well-trained hitman who is now killing his former customers. He isn't working for an organized crime syndicate for the murders; instead, all of the victims hired him through an online service. Hitmen do not leave a message unless they are paid to or they are trying to intimidate or send a message to their clients or competition. The unsub's message of painting inverted pulcinella masks on his victims' faces is a warning against betraying the trust between a client and their hitman. He has suffered a massive injury to the mouth or voice, and this would normally bring attention on him, but because no one has spotted him yet, the unsub likely has deep pockets and a network of resources at his disposal.
Montolo is based on Giuseppe Greco. Like Montolo, Greco was a prolific Italian hitman who shared the same first name, survived a gunshot wound during an attempt on his life, were deeply involved in criminal syndicates through family connections, and was later murdered by a fellow hitman.
- Montolo seems to be inspired by Gwynplaine, a character who appeared in the 1928 silent film The Man Who Laughs. Both had facial scars that they covered (along with the lower half of their faces) with a black high collar. In addition, Montolo's temporary inability to speak seems to be an allusion to the film's status as a silent film.
- Montolo is partially similar to John Myers, a.k.a. "The Silencer", who appeared in Season Eight in the sense that both were infamous serial killers who were unable to speak due to some form of disability, though Montolo's inability was temporary.