Grisha Alexandrovich Nikolaev or G. Callen (G. or Callen to his NCIS colleagues) is an NCIS Special Agent and also a member of the Office of Special Projects in Los Angeles.
Born in Romania to a Russian father, KGB Major Nikita Alexsandr Reznikov and Clara Callen, a half-Roma CIA agent as well as having a sister named Amy, Callen lived a relatively normal life up until the age of five when his mother sought to escape to the United States along with her two children to avoid being targeted by members of the Comescu family who had long sworn since a blood oath to eliminate any and all traces of the Callen family.
Waiting at a beach, young Callen played in the sand and was subsequently distracted by a man handing him a toy soldier while another man, a second member of the Comescu family shot Clara dead in cold blood with Callen suppressing the memory from his subconscious for many years.
Eventually, both young Callen and Amy were taken from Romania and brought to the United States. The two siblings were split up and subsequently placed in different orphanages.
In 1975, at the age of five, Callen withdrew from the orphanage and was placed in the foster care system instead. He did not have an ideal upbringing as he went through over thirty-seven foster homes, presumably experiencing various problems in the foster homes he was in which in turn left him emotionally damaged and unable to trust anyone whatsoever.
Upon turning eighteen, Callen enlisted in the United States Army although his history and what rank he held remain unknown.
After that, Callen joined the CIA, working alongside Trent Kort.
Callen then left the CIA and joined the DEA- Drug Enforcement Agency and then the FBI before choosing to join NCIS.
After becoming an NCIS Special Agent, Callen befriended and met fellow NCIS Agent Leroy Jethro Gibbs with two assignments bringing both men to Serbia and Russia with Gibbs saving Callen's life on both occasions in the two countries.
At some point, Callen was reassigned to the Office of Special Projects located in Los Angeles where he remains to this day. He also presumably worked alongside a few partners before working alongside his current partner, Sam Hanna.
NCIS Season 6
Callen made his debut appearance in the NCIS Season 6/NCIS: Los Angeles backdoor pilot episode, "Legend Part 1" where he was seen ignoring a call from his boss, NCIS Special Agent Lara Macy before he spoke to his old friend, fellow NCIS Special Agent Leroy Jethro Gibbs.
The two spoke for a while about the past they had shared together although they were interrupted by a young Russian woman cycling by.
After they had finished, Callen left to go get ready for the second undercover operation he would play a main role in.