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G. Callen is the main protagonist of the CBS series NCIS: Los Angeles, which is a spin-off of the original NCIS. He is portrayed by American actor Chris O'Donnell.
Callen is the NCIS Special Agent in Charge of the NCIS Office of Special Projects team stationed in Los Angeles, reporting directly to the OSP Operations Manager Hetty Lange. He and his friends/teammates were introduced in a backdoor pilot, "Legend" from NCIS. He and Lange first met three years prior to the start of the series, as revealed in the season one episode "Identity".
While his friends address him as "G", Lange refuses to do so, saying, "It's not a name, it's a letter." She instead addresses him as "Mr. Callen" ("Identity"). Callen once served with his good friend Leroy Jethro Gibbs, who now works at the NCIS Headquarters located in the Washington Navy Yard. Callen specializes in deep undercover work. In addition to English, Callen is fluent in at least six other languages: Spanish, Polish, Russian, German (with an English accent, which is claimed to be Austrian), Italian (with a Northern accent), and French. Callen also claims to speak Czech and Romanian; furthermore, his Russian is arguably good enough to be taken for one, but not the other, Chechen dialect.
The fake death certificate shown by Hetty to Alexa Comescu in the season two finale states that his date of birth is March 11, 1970.
In season two, Episode 13 "Archangel", Callen states that the first time he fired a gun was at the age of 20. In the same episode, it was revealed that Hetty has beaten him on a race up the NCIS rock wall three times in a row, and Callen refuses to challenge her to a re-match despite goading from Sam Hanna.
Callen had a troubled past. In the episode "Pushback", it is revealed that Callen lived in 37 foster homes starting from the age of five. He sometimes moved every few days, and the longest he ever stayed in one place was for three months at fourteen, when he lived with the family of a Russian girl named Alina Rostoff (who he called "little sister"). It is revealed through flashbacks in the season two finale that Callen spent at least part of his childhood on the Romanian coast of the Black Sea. It is revealed in "Identity" that he is called "G" not because he dislikes his first name, but because he does not actually know what it stands for, as the system never told him.
It is later revealed that as an adult, his foster-sister Rostoff was secretly sent to watch Callen and to prevent him from being killed by Russian mobsters. Callen is fluent in Russian due to his former relationship with the Rostoff family. Throughout the first season, it is shown that Callen is homeless, staying temporarily with his various NCIS colleagues and sometimes in the Operations Center itself.
In the season two premiere "Human Traffic", Hetty makes arrangements for Callen to purchase a house of his own—the very same house where he lived with Alina Rostoff as a foster child.
At the end of "Legend", Callen was almost killed in a drive-by shooting. Callen and the team later realized (in the first season episode "Pushback") that his shooting was related to a mission Callen took part in against the Russian Mob, as a DEA agent ten years earlier.
At some point prior to joining NCIS Callen was an officer of the Central Intelligence Agency. During this time he was partnered with Tracy Rosetti, whom he later married while undercover using the surname Keller. Following a mission in which it is implied that she placed the success of the operation above both their professional and personal partnerships, the two divorced. In the settlement Tracy received custody of Buddy, their dog.
In the first season finale "Callen, G.", Callen tracks down a woman named Amy Taylor (née Callen) who initially claims to be his older sister, but then admits her real name is actually Hannah Lawson, and that she was friends with the real Amy Callen when they lived in an orphanage together. The real Amy had died at the age of 11, when the two friends sneaked out at night to play in the nearby river and Amy had been swept away in an accident. When Hannah returned, she slept in Amy's bed, and the authorities there never realized what had happened. When Amy's body was found a month later, she was buried under Hannah's name as the authorities didn't know any better. Callen later goes to visit his sister's grave, and finds flowers and a recent note left "For my Sweet daughter always in my Heart". Also during the episode, Nate Getz discovers that a list of all of Callen's foster homes and orphanages was written by one person, albeit over a number of years and with different pens and pencils, indicating that someone had been keeping an eye on Callen during that time. The same list showed the various orphanages and foster homes where the real Amy lived until her death, indicating that this person also knew about the switch between Hannah and Amy. In "Deadline", Callen discovers that the list was written by Hetty.
The second season mostly revolves around Callen, who has been stalked by someone he does not know. He sees someone near the grave of his sister, and over the course of the season, runs into people who seem to know more about his past than he does. When Hetty resigns and takes off to Prague, Callen finds out from Director Vance that Hetty has started her own investigation, Operation Comescu, which is all about Callen. Callen and the team then resign too when Vance refuses to fly them to Prague to get Hetty.
Arriving in Prague, the team discovers that Hetty has been taken by the Comescus, and that the Comescus have been in a decades-long blood feud with the Callens, of which G is the last one. The team tracks their safe house down to a house on the beach, and Callen starts to remember things from his past, as he realizes he has been on this beach before. Inside the house, Hetty convinces Alexa Comescu, the matriarch of the Comescu family, that Callen died three weeks ago, showing her a clip of the attempted assassination and a death certificate she had made.
In the third season premiere, "Lange, H.", it is revealed that Callen's grandfather was an OSS agent stationed in Romania. In the third season episode #4 'Deadline', Hetty Lange reveals to Callen that his Grandfather was named George Callen ('G'?), and that he parachuted into Romania during the War, and at the end took part in hunting those guilty of war crimes, he subsequently found and killed several members of the Comescu family. In 1947 he met and married a 'Roma Girl' (Callen's grandmother, name unknown). George Callen was murdered by the Comescu family. The remainder of Callen's family (his Grandmother and her daughter, Callen's Mother, Clara) fled to America until G's mother was recruited by the CIA. She returned to Romania, then a year later vanished for six years. The Comescu family had not forgotten their grudge, and came looking for the Callens. One of Callen's most vivid memories of his childhood is of being given a toy soldier by a stranger while he played on the beach. He realizes that the beach is not in California as he had always assumed, but on the Black Sea coast, and that the toy was given to him to distract him while a Comescu assassin killed his mother. It is never explained why the Comescu family let Callen and his sister live, though Alexa Comescu describes it as "a mistake". Despite Alexa's claim that she knows everything about him—including what his initial stands for—Callen does nothing to protect her and allows Hunter to kill her.
Back in LA, Hunter is the acting Operations Manager until Hetty returns to work after sustaining a gunshot. Callen does not trust her and wants to know more about himself and what was on the laptop that she retrieved at the Comescu house. Hunter later tells him that all the contents of the laptop were concerning the family's illegal activities and not about Callen. Hetty later returns when Hunter leaves for another undercover operation in Europe and offers to tell Callen everything starting with the reason she was trying to protect him: she had failed to protect his mother. She then gives him his mother's name: Clara. She also tells him that his mother was a CIA agent and that Hetty had been her handler when Clara had gone back on assignment to Romania. After a year of mission, Clara vanished for six years and resurfaced with two children, desperate to get out of Romania. Hetty had been sent to meet her on the beach where the Comescus killed her. She also says that she doesn't know how he and his sister came to the U.S. and that she had no idea who his father is but that she was the one responsible for getting him out of the orphanage and into foster homes.
In season three Episode 14 "Partner" it is stated that Sam and Callen have been partners for more than five years.
In the third season finale, "Sans Voir", Agent Mike Renko and former Operations Manager Lauren Hunter are both murdered at the hands of "The Chameleon". Callen responds by gunning down "The Chameleon" following a prisoner exchange and is arrested by the local police. The episode ends with Callen being arrested by LAPD and Hetty resigning from her position at OSP, although a series of flashbacks in the season four opening episode "Endgame" reveal that the Chameleon's death was actually faked. After the Chameleon and Callen are traded back to their respective sides in a prisoner exchange, Callen, following his suspension, presumably returns to lead the OSP team once again.
In the season five episode "Reznikov, N.", a man named Michael Reinhardt appears, who claims to be actually named Nikita Alexsandr Reznikov, Callen's father. He is kidnapped and killed by one of the surviving members of the Comescu family, Vasile. At the end of the episode, Hetty and Arkady Kolcheck explain to Callen that Reznikov was actually a KGB major who was arrested and sent to a Soviet gulag in 1974, around the time Callen and his sister came to America. Reinhardt (actually named Hans Schreiber) was a former East German who owed Reznikov a favor, and repaid it by keeping track of Callen and his sister. The Comescus mistook him for Reznikov, who kept the ruse to protect Callen. In Reinhardt's house, Deeks discovered an 8mm film of Callen and Amy playing with Reznikov, which Hetty then gave to Callen.
In "War Crimes", Sam, Michelle and Hetty set Callen up on a blind date with Joelle Taylor, a friend of the Hannas' and their daughter's former kindergarden teacher. Both unaware of the trick, they nevertheless get on well and, throughout season five, Joelle is mentionned as Callen's girlfriend. In "Humbug", an armed robber threatens Joelle in public after stealing some malware from a defense contractor. When it is learnt that the robber placed the stolen malware on Joelle, Callen is forced to put himself in danger to protect her, eventually being forced to reveal to her that he is an NCIS agent. Although hurt by this revelation of Callen's deception, she eventually accepts to continue their relationship as long as he tells her more about himself. Callen and Joelle are confirmed to be still together by both Sam and Callen himself.