Special Agent G. Callen

Special Agent G. Callen is a main character on the series NCIS: Los Angeles which itself is a spin-off of the original "NCIS" series.

Callen is also an NCIS Special Agent who is currently assigned to the Office of Special Projects branch in Los Angeles, California and is currently partnered with Special Agent Sam Hanna.

In addition to being an NCIS Special Agent, Callen is also in charge of the NCIS: Office of Special Projects field team currently composed of himself, Sam, Special Agent Kensi Blye and NCIS/Los Angeles Police Department Liaison Officer, Detective Marty Deeks with support coming from Analyst Nell Jones and Tech Operator Eric Beale with the entire OSP being overseen by Operations Manager Henrietta Lange.

Pre-Series
Born in Romania to an unnamed father and Clara Callen, a CIA agent, young G. Callen lived a relatively normal life up until the age of six or seven.

The Comescu family who Callen's grandfather and possibly Clara's father had killed many members of later retaliated by killing Callen's grandparents before their daughter, Clara escape to the United States. 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 solider.

At some point, young G. Callen and his sister, Amy were brought to the United States with both children being placed in foster homes. Amy died when she was very young while Callen grew up as part of the foster system.

Upon turning eighteen, Callen presumably joined the Army while working for the CIA in his later years. He then joined NCIS where he had assignments which had him working with Gibbs with the tasks 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 where he remains to this day.

NCIS Season 6
Like Lara Macy, Eric Beale, Kensi Blye, Nate Getz and Sam Hanna, Callen made his first appearance in the NCIS Season 6/NCIS: Los Angeles backdoor pilot, Legend Part 1.

Callen was first seen talking to Gibbs in a small park located presumably on Venice Beach with the two agents remembering the times they'd spent together in the past.

In Legend Part 2 (episode), Callen went undercover as arms dealer, Liam Coyle and was able to infiltrate the main terrorist cell which was located in a car shop in downtown Los Angeles.

A few hours later and after the case had been solved, Callen was left badly wounded in a surprise drive-by shooting that Sam also witnessed.

In Semper Fidelis (episode), Callen was revealed to still be in critical condition and according to Gibbs, "hanging in there" with even FBI agent Tobias Fornell asking about Callen.

NCIS: Los Angeles Season 1
Four months later Callen returned to his duties where it was later revealed that a former CIA operative Ethan Stanhope arranged the assassination attempt. Stanhope desired to eliminate all persons with knowledge of Operation Cossack a joint NCIS/CIA operation in Russia that failed when it was compromised by Stanhope. Stanhope's goal was to conceal his treasonous actions and prevent any adverse disclosures that would have harmed his business ventures

NCIS: Los Angeles Season 3/Hawaii Five-0 Season 2
At the request of Detective Danny Williams, Callen and Sam arrived in Hawaii to help the H50 team track down a known criminal named Dracul Comescu who was a person of interest of the OSP team as Comescu's own family had killed Callen's family in a feud that had lasted for decades with Dracul and G. being the solo survivors of the feud after Callen, Sam, Kensi and Deeks had killed the remaining members of the Comescu family, leaving Dracu as the only surviving Comescu.

Upon arriving in Hawaii, Callen and Sam formed new friendships with the Hawaii Five-0 team that Danny was temporarily leading due to the fact that the normal team leader, Lieutenant Commander Steve McGarrett was out of the country.

In the field, Callen was paired with Lieutenant Chin Ho Kelly while Sam worked with Danny.

Eventually, in the International Market, Comescu and Callen confronted each other with Callen being forced to shoot Comescu just as Comescu was able to unleash a viral containing smallpox which also resulted in the long-standing feud between Callen and Comescu being settled for good although the team's joy at solving the case was short-lived as Officer Kono Kalakaua called Danny and told him that the vials they had retrieved at the scene didn't contain smallpox, meaning that someone had already switched out the vials and that the person in question, Dr. Jarrod Prodeman had already gotten to Los Angeles, forcing Callen and Sam to return to LA to stop the smallpox outbreak with Danny and Chin coming along to help them.

Trivia

 * Despite his real name is revealed, he still being referred 'G' or 'Callen'. Probably because his real name was too long or too weird. A running gag in the series, he will be makin fun by people's or his neighboor because it's too 'girly'.