"Why did things have to change?"
As a child, Caleb was a child prodigy who loved chess. During a chess tournament, he met Harvey Morell, whom he defeated and later befriended, becoming extremely close to him. Harvey was also a child prodigy who shared Caleb's love of the game and also his interest in Roman history, animal dissection, and crime journalism; one of the crimes they obsessed over was the infamous Zodiac Killer. They kept a scrapbook filled with newspaper clippings about his murders and wrote about him in their junior high newspaper under their column, Crime Beat. On December 23, 2000, while still in junior high, Caleb and Harvey killed a seven-year-old boy named Robbie Shaw together, taking him to the woods and strangling him. While Caleb was the one who killed Robbie, Harvey was the one who chose him to be murdered because Robbie's older brother had picked on him. They then buried the body in Harvey's backyard and got away with the murder since the body was never found. They remained best friends until after high school, when they drifted apart and Harvey caught the attention of Marisa Devon, whom they both had fallen for in junior high. Harvey and Marisa started dating, developing a steady romantic relationship, and eventually got engaged to be married. Caleb, still in love with Marisa and resentful that Harvey was leaving him after all they had been through, became infuriated, which culminated in 2012 when he decided to copycat the Zodiac in a desperate attempt to become closer to Harvey again.
Caleb selects a couple, David Atley and Nicole Puli (the latter of whom resembles Marisa), following them to a park at Marin County. Waiting for park service to get far from the couple, Caleb then ambushes David and Nicole in their car, killing both before leaving behind a symbol of the Zodiac's, a photo of one of the Zodiac's alleged victims, and a shirt pressed to look old with the stains of blood that matches that of the Zodiac's last victim (taxicab driver Paul Stine) type-wise. The next day, he plays a game of chess with a workmate while on break from his job as a trash collector; he emerges victorious. Caleb immediately calls Harvey, but the phone goes to voice-mail, infuriating Caleb. He then sends the police a cipher in the form of a spam message before attending a party celebrating Harvey and Marisa's engagement and tells everyone of the history of his and Harvey's friendship. Impressed, Harvey thanks Caleb for the presentation, and Caleb invites him over for a bachelor party in Las Vegas, but he turns down the request due to a tight schedule. However, Marisa gets Harvey to reconsider.
Reid is able to translate Caleb's cipher, leading the BAU to Golden Gate Park, where they believe is the place the unsub will strike next. Using decoys, the BAU lure out what they think is the unsub, only for the man to be identified as a messenger for Reid, who gives him a letter sent by Caleb. The letter reads, "You're not as smart as you think you are." Meanwhile, Caleb ambushes another couple taking an evening stroll at Albany Park, stabbing both to death. He then meets up with Marisa at Chinatown, thinking she wants to call off the wedding, only to find out that she just had a minor fight with her mother and wants to talk to him about how to prepare a surprise for Harvey, a man room to be built while they are on their honeymoon. Agitated, Caleb tells Marisa that Harvey is weird and that he would not like a man room and that she doesn't know Harvey at all. Caleb tells her that he wants her to like him, and Marisa replies that she does indeed like him, but as a friend, before leaving. Snapping, Caleb follows Marisa and attacks her when she boards a taxicab, shooting the driver in the same manner as the Zodiac's murder of Paul Stine before abducting Marisa and tying her up, leaving her at an abandoned Dragon Temple restaurant.
Caleb then approaches Harvey and convinces him to play a childhood game where they would "profile" the Zodiac Killer. The two visit Caleb's previous murder sites, Harvey theorizing about the Zodiac all the while. Eventually, the two end up standing at the door of the Dragon Temple restaurant, where Caleb previously worked at; they go inside, where Harvey finds Marisa. Realizing that Caleb abducted her, Harvey berates him for doing so as he unties Marisa. Angered, Caleb pulls out his gun and fires a shot at the floor as the two try to leave, stopping both in their tracks. Caleb tries to reconnect with Harvey and reminds him of how they murdered Robbie Shaw, a fact that shocks Marisa. The BAU, having discerned Caleb as the unsub, storm into the restaurant and Reid holds Caleb at gunpoint alone. Reid tells Caleb that he believes Harvey was the mastermind behind Robbie Shaw's murder, then reveals that Harvey had been planning to move to Shanghai without telling him in an attempt to get away from him. Horrified by the revelation, Caleb tells Reid where Robbie Shaw's body is buried and drops the gun, allowing the BAU to arrest him and Harvey.
Caleb copied the real Zodiac's M.O. as minutely as he could, killing his victims either by shooting them with a 9mm handgun or by stabbing them with a knife. He also mimicked the Zodiac's overkill on his female victims, though Caleb's overkill was much more aggressive than that of the Zodiac's. One notable difference between his murders and those of the Zodiac was that his female victims were all Caucasian brunettes in their mid-20s (surrogates for Marisa). Other differences were that his cooling-off period between murders was significantly shorter than the Zodiac's (due to Harvey and Marisa's upcoming marriage) and was much more organized during the Ferrin-Mageau and Hartnell-Shepherd-based attacks, never leaving any survivors as the Zodiac did in both attacks. In addition, he did some alterations during his killings, in comparison with those of the Zodiac, usually for unknown reasons:
- During the Marin County shooting, he fired fifteen times, with two hitting David Atley and the last thirteen hitting Nicole Puli. The Zodiac only fired nine times during the Ferrin-Mageau attack, with both victims being hit an approximate equal amount of times.
- During the Albany Park stabbings, he first slashed Alec Wyseck's throat before stabbing him. The Zodiac did not do this to Bryan Hartnell when he attacked him. He also didn't stab Tara Henson in the back, just as the Zodiac did with Cecelia Shepherd.
- During his killing of the taxicab driver, he shot him outside his taxi. However, the Zodiac killed Paul Stine while he was inside his taxi. This may have been because Marisa was inside the taxi at the time of the killing. In addition, Caleb did not seem to tear away a piece of the man's shirt as the Zodiac did with Stine.
Caleb based the locations of his murders after a famous game of chess between Boris Spassky and Bobby Fischer. Also, not unlike George Foyet (who was based on the Zodiac), Caleb sometimes took items from his crime scenes and left them on the next.
He went to great lengths to give the impression that he was the actual Zodiac Killer from the 1960s, even wearing the same kind and size of shoes that the Zodiac wore as well as a hooded shirt identical to the one witnesses described; at his first crime scene, he left behind old photos of a suspected Zodiac victim, which he had stolen from an old police detective who worked the original case. When he killed Robbie Shaw with Harvey, Caleb lured him into a forest, strangled him, and then buried his body in the backyard of Harvey's house.
The unsub is a physically fit white male in his 20s to early-30s. Though he is highly intelligent, he likely holds a menial or low-level job. He has few friends and fantasizes about a world where he can feel special. Unlike the real Zodiac, his goal isn't mass hysteria, but his issue is with an individual woman with whom he is obsessed. The women he is killing are likely surrogates for this particular woman; he has been rejected by her or simply believes he cannot have her. Consequently, he hates her and every man she is with. His crimes may be an attempt to impress her or prove his love for her. He has chosen to copy the Zodiac because he wants to feel the same kind of brilliance, power, and fame the original had felt. The fact that his breaks between murders are getting shorter suggests that he may be losing the object of his obsession.
Caleb seems to have been inspired by Heriberto Seda, a well-known copycat of the Zodiac Killer. Like Seda, he was deeply fascinated by the Zodiac, copied his M.O. (though to a greater extent compared to Seda), kept scrapbooks of his crimes, and attempted to give the impression that he was the original Zodiac.
Also, him copying the Zodiac aside, Caleb and Harvey's way of killing a child may have some basis in the murder of Bobby Franks by Nathan Leopold and Richard Loeb. Much like Caleb and Harvey, Leopold and Loeb killed their victim just to see if they could get away with it. The members of both pairs were also highly intelligent and bonded over mutual interests, particularly that of crime.
The pattern of his crimes also seems to be indirectly based on serial killer Edward Edwards, who shot a couple in 1977 and stabbed and strangled another in 1980; Edwards' M.O. also involved the strangulation of one of the victims as well as stabbing. It should be noted that Edwards was living in northern California at the time of the Zodiac killings and was even possibly similar to a physical description of the killer, though these claims were disputed.