Grisha Aleksandrovich Nikolaev-Callen, was born on March 11, 1970 in Romania to Nikita Aleksandr Reznikov, a Major in the KGB, and Clara Callen, a CIA officer. Young Callen lived a relatively normal life up until the age of 5.
Clara's father had killed many members of the Comescu Family, which led to a blood feud between the two families. In retaliation, the Comescu family killed Callen's grandfather, George Callen, and Clara fled to the U.S. with her two children, Callen and his sister, Amy. Unfortunately, the Comescus still held a strong, undying grudge against the Callens, and an assassin working for the family later killed Clara while another man distracted young Callen by giving him a toy soldier. Hetty Lange, a member of the CIA who had been Clara's handler, was sent to help Clara and her children, but she was too late.
Callen and Amy were brought to the U.S. after the death of their mother and were placed in foster care in separate homes. Amy spent most of her time in an orphanage, while Callen constantly went from one foster family to another. During her childhood, Amy snuck out with one of the other girls from the orphanage to a nearby river, where she accidentally fell in and drowned. Hannah, the girl she was with at the time, could not save her, and when she went back to the orphanage, she slept in Amy's bed and took on her name. Hannah was able to keep the whole thing a secret, until Callen discovered he had a sister and went looking for her, only to find Hannah instead. She later explained everything to him about his sister.
Callen was not aware he had a sister until Hannah Lawson told him so. Over time, as he moved through the foster system, he eventually forgot about her, as they had not seen or heard anything about each other in years.
While Callen was a teenager and began to continuously get in trouble, Hetty Lange rescued him and took him under her wing. She worked with and trained Callen. At some point after meeting Hetty, he worked for the CIA, DEA, and FBI before joining NCIS. After joining NCIS, he had various assignments, one of which brought him to Serbia and Russia where he met NCIS Special Agent Leroy Jethro Gibbs. Some time after this, Callen was assigned to the Office of Special Projects in L.A. where he currently works.
After a few years working with NCIS, Callen wanted to know more about his family, and wanted to know what the "G." from his first name stood for. No one in the many foster homes he had grown up in knew what it meant, so he had lived his whole life without a first name. Hetty had been protecting Callen from the Comescus his whole life because they were looking for him and wanted to kill him as part of the continuation of the feud. Hetty knew that if Callen found out about his family and the Comescus killing his mother, Callen would go back to Romania to find out more about his family and may be killed in the process. When the secret was close to being revealed, Hetty resigned in order to go back to Romania and try to make the Comescus believe that Callen had been killed in a shooting, which had taken place during the NCIS/NCIS: Los Angeles Crossover Episode. She presented them with a false death certificate, but the head of the Comescu Family did not believe her, and Hetty was wounded in the process of being rescued by Callen, Sam, Deeks, and Kensi.
In the 100th Episode of NCIS: Los Angeles, a man pretending to be his father Michael Reinhardt was kidnapped, where a message was left at the scene: "Callen we have your father." Throughout the episode, many pictures of Callen from when he was younger were shown. One of the pictures sparked an old memory and a conversation with Kensi and Deeks, where Callen revealed that he used to garden with one of his foster mothers, but his foster father used to beat him over the head with a broom handle. He revealed that he was moved from the foster home not because of the father's abuse, but because one day Callen took the broom handle from him and broke it over his foster father's head. It was revealed at the end of the episode that Michael was not truly Callen's father, but Callen learned his father's name: Nikita Aleksandr Reznikov.
NCIS Season 6
However, at the end of Episode: Legend (Part 2), Callen was wounded in a drive-by shooting that Sam also happened to witness and in the next Episode: Semper Fidelis, was described as being in critical condition.
NCIS: Los Angeles Season 1
In the Episode: Pushback, it was revealed that a former CIA Officer Ethan Stanhope arranged the assassination attempt. Stanhope desired to eliminate all persons with knowledge of Operation Cossack, a joint NCIS/CIA/DEA operation in Russia that failed when it was compromised by Stanhope. Stanhope's goal was to conceal his treason and prevent any adverse disclosures that would have harmed his business ventures. Despite Stanhope's best efforts, Callen, backed by Sam and Kensi, succeeded in arresting him, ending the case for good.
In the Season One Finale, Callen tracks down a woman named Amy Taylor (ńee Callen) who initially claims to be his sister, but then admits her name is actually Hannah Lawson, and that she was friends with the real Amy Callen when they lived in an orphanage together. Amy died at age 11 when they snuck out at night to play in a river, and Amy was swept away. When Hannah returned, she slept in Amy's bed, and the authorities never knew what 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 note left "For my Sweet Daughter always in my Heart". Also during the episode, Nate discovers that a list of all of Callen's foster homes and orphanages was written by one person, albeit over a number of years, indicating that someone had been keeping an eye on young Callen. The same list showed the various orphanages and foster homes where the real and fake Amy, including the last foster home where Amy (aka Hannah) stayed for many years, indicating this person did not know about the switch between Hannah and Amy. In Episode: Deadline, Callen discovers that Hetty had kept an eye on him during his youth (in the Episode: Rage), and also knew his sister died. But in Episode: Reznikov, N. it is revealed that Hans Schreiber also kept track of Callen as a favor to Callen's father. It has not been confirmed who actually wrote the list of foster homes and orphanages; however, it is assumed that he wrote the list as Nate analyzed the list as having been written with an aging hand, as and when the changes of homes took place.
NCIS: Los Angeles Season 2
Season Two 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 OSP Team then resign when Vance refuses to fly them to Prague to get Hetty. Arriving in Prague, the OSP 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, one of which Callen is the last. The OSP 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 there before. Inside, Hetty fails to convince Alexa Comescu, the matriarch of the family, that Callen died 3 weeks ago, showing her a clip of the assassination attempt and a false death certificate.
NCIS: Los Angeles Season 3
NCIS: Los Angeles Season 4
Callen goes to Iran to capture Marcel Janvier aka “The Chameleon”. He forces him to set up a meet with a Russian arms dealer, Isaak Sidorov. The Chameleon gives up Callen's partner, Sam. Callen is furious and sends Kensi and Granger to find Sam.
NCIS: Los Angeles Season 6
Callen and Sam are trapped in and underwater submarine that is headed for an aircraft carrier. The sub has been made into a torpedo and they are going to explode with the sub. They bust a hole in the sub to make their captors open the locked compartment they are trapped in. They then kill their captors and escape. The Navy blows up the sub just as Sam and Callen escaped. They blew it up because there were terrorists aboard.
NCIS: Los Angeles Season 8
When Callen is not solving cases or covering with a mole inside the OSP, he is dealing with a problem he never expected to have: a living family. His father, now in the U.S., had another lover after Grisha and Amy's mother, and she had a daughter, Alexandra Reynolds. Despite being a company based in a mole search. That ended up failing Alex's ex, who first knows his son's grandfather when the older man abducts him. This is a strain on Callen's relationship with Nikita. This is partly because Callen has to give up a professional integrity for Nikita to jail and partly because his secrets, first approach to interpersonal communication between himself, separated from Alex and his son; do not work. To start a relationship with Alex, Callen and Nikita appear at Alex's house. When Jake then says the oven has subsided, Alex invites them both to dinner.
NCIS: Los Angeles Season 9
In Season Nine, sometime following the events of the Season Eight Finale, Hetty has disappeared, apparently having retired from NCIS for good and sold most of her properties and taken her boat. Following Hetty's departure, Executive Assistant Director, Pacific Operations Shay Mosley appears to take her place and oversee the team. Throughout the season, Callen and the team are concerned regarding Hetty's disappearance and begin to try and locate her. Eventually, it’s revealed Hetty traveled to Vietnam to rescue Harris Keane, a former member of her CIA Unit during the war that Hetty thought was dead but later finds herself captured by a human trafficker and criminal Dang. Hetty’s survival was confirmed by a joint operation combining the OSP team as well as Hetty’s former CIA Unit in Vietnam, with Keane and Hetty both making it out of Vietnam. Hetty resumes her duties as Operations Manager, but finds difficulty working with Shay Mosley who intends to break up the team.
In the Season Nine Finale, Callen, along with the rest of the team, learn the truth about Mosley's kidnapped son and Callen's promise to help her when they come into contact with a suspect on a current case who is connected to Mosley's former lover, Spencer Williams. Upon learning this information, Mosley sees this as her best chance of finding and getting her son back. Mosley and the team learn they are in Mexico, but the off-the-books Mexico mission to rescue Mosley's son was highly dangerous, putting the team at risk. Even though Deeks is fired by Mosley after confronting her about her unfair, reckless, and disrespectful behavior, Deeks still joins the team on the off-the-books Mexico mission (unsanctioned) to save Mosley's son. However, while the mission is successful, reuniting Mosley with her son and returning them to LA, the rest of the team are struck by an RPG while escaping in their SUV, leaving uncertain who is dead or alive.
NCIS: Los Angeles Season 10
Shortly after the Season Nine Finale, Callen and the team are revealed to have survived the impact that hit their SUV. Kensi is the least injured member of the team, but becomes extremely worried when Deeks remains unconscious following the explosion. The team are forced to separate so Callen & Sam take a dirt bike as far as it can go. After the bike cuts out, they walk through the desert to find safety and shelter. Eventually, Callen & Sam run into a kid who leads them to shelter then finds help, the kid came back with his parents who manages to get to a hospital. Callen & Sam regroup with Kensi, Deeks & Turk and while Callen, Sam & Deeks are in the ER, Kensi & Turk stand guard outside and get ready for a fight from Spencer Williams’ men who are hunting them. At the last minute, Mosley confronts Williams with duel wield pistols aimed at him telling him “Stand Down” after telling Mosley “No” she shoots him and tells his bodyguard to call it off because now that he’s dead no one's getting paid so they did. Afterwards, the calvary that Hetty sent arrives and evacuate the team to Balboa Naval Hospital.