Detective Marty Deeks

Marty Deeks is a Detective with the Los Angeles Police Department and also an LAPD liaison officer attached to the NCIS Office of Special Projects in Los Angeles. He becomes a permanent member of the NCIS team very soon, and Is there to this day.

Pre-Series
The son of a man named Gordon John Brandel and Roberta Deeks, Deeks was born on January 8th, 1979 in California but Deeks didn't have a happy childhood as his father was an alcoholic who frequently abused Deeks and his mother.

One day, Brandel became so drunk that he started wielding a shotgun at Deeks and his mother. In response, young Deeks shot Brandel non-fatally with Brandel later ending up in prison.

Deeks later presumably moved away and attended college before choosing to join the Los Angeles Police Department.

NCIS: Los Angeles Season 1
Deeks appeared in the episode, "Hand-to-Hand", undercover as a mixed martial artist named "Jason Wyler". He managed to get into a local MMA gym that served as front for a drug smuggling operation. The Office of Special Projects targeted the gym after Daniel Zuna, a Marine on leave was found murdered in an alleyway outside a nightclub.

NCIS Special Agent Sam Hanna went in undercover, using the alias, "DeMarco Williams" with Sam fighting Deeks and knocking him out.

During a subsequent encounter, both of them were taken into custody to preserve their covers. It was only after being released from the holding cells that he was revealed to be a Detective with the LAPD. Detective Deeks saved Agent Hanna's life after his cover was blown.

Deeks was later seen in a bar, drinking away his sorrows with a cup of coffee while still smarting from the tongue-lashing he had received from his boss. Hetty later arrived at the same place and, after revealing that she too had suffered the same treatment from her boss, told Deeks that she had decided to create a liaison position between the LAPD and NCIS. Deeks was in disbelief at first but upon Hetty offering him the position, having told Deeks that he was the suitable candidate for the job, Deeks accepted.

Deeks joined the team in "Fame" to help them solve a case before leaving NCIS to go take part in a deep undercover operation despite the fact that some of the team, including Sam were hostile to him given that Deeks's predecessor, Agent Dominic Vail who was still missing had not returned to the team.

During a mission to save Kensi, with Hassad Al-Jahiri weakened by a gunshot from Callen, Deeks took the opportunity to finish Hassad by dropping to the ground and then firing a single shot which killed Hassad instantly. Deeks then left the team to go on a deep undercover mission and was only mentioned once in the Season 1 episode, "Burned".

NCIS: Los Angeles Season 2
In the Season 2 premiere episode, "Human Traffic" after his undercover mission went south, Deeks contacted Hetty for help and begged for her to assist in his mission which involved underage young girls. He officially became Kensi's partner and deceased Agent Dominic Vail's replacement in the next episode, "Black Widow", resulting in him joining the team permanently albeit as a liaison officer.

NCIS: Los Angeles Season 3/Hawaii Five-0 Season 2
In "The Debt", Deeks's position was terminated and he was fired from NCIS after unintentionally shooting an unarmed man. It was later revealed that the supposedly dead victim was actually an undercover NCIS Agent working out of Washington D.C. The incident gave the LAPD a reason to take Deeks back after so long while also cutting their ties with NCIS. The OSP team were eventually able to find out the identity of the mole and in the end, Deeks returned to the OSP, having been reappointed as the LAPD/NCIS Liaison Officer.