Originally a temporary replacement for Ducky's original assistant, Gerald Jackson, Palmer later became the permanent Medical Examiner Assistant after Jackson resulted from NCIS for good in the aftermath of Jackson's own kidnapping at the hands of terrorist Ari Haswari with Palmer holding the role to this day.
NCIS Season 1
Palmer made his debut appearance in the Season 1 episode, Split Decision (episode) where at the time of his debut, he was a temporary Medical Examiner Assistant, having been chosen to replace Gerald Jackson, Ducky's current assistant who had been left injured after being shot by an unidentified terrorist in the Season 1 episode, Bête Noire (episode).
NCIS Season 2
In The Good Wives Club (episode), it was revealed that Palmer suffers from claustrophobia.
NCIS Season 3
NCIS Season 4
In Season 4, it was revealed that Palmer was sleeping with NCIS Special Agent Michelle Lee and that Palmer also has mild diabetes which he regulates using insulin.
NCIS Season 5
In Dog Tags (episode), Palmer spent two summers working as a veterinary assistant which proved essential when he stitched up the wounds that a former drug dog, Butch had received after Agent Timothy McGee had shot the dog in self-defense. It was also revealed that Palmer had spent two summers as a veterinary assistant during college.
In About Face (episode), Palmer was left shaken after being shot at while alone at a crime scene. It was revealed that his mother's first name was Eunice and that he had a brother named Clark.
NCIS Season 6
It has been revealed that in addition to going to medical school, Palmer has hopes of becoming an NCIS Special Agent as seen in the episode, "Bounce", when after Tony departs, Palmer puts on an NCIS cap and says, "That's Agent Palmer".
NCIS Season 7
In Mother's Day (episode), it was shown that Palmer was dating a woman named Breena Slater who works as an embalmer.
NCIS Season 8
In "Pyramid", Palmer was one of the three people along with CIA agent Trent Kort and NCIS Special Agent Erica Jane Barrett to be taken hostage and tortured by serial killer, Lt. Jonas Cobb but after Cobb's death at the hands of both Leroy Jethro Gibbs and Leon Vance, all three were rescued.
NCIS Season 9
NCIS Season 10
In Extreme Prejudice (episode), it was revealed that Palmer was the one who had discovered Ducky on the beach after the latter had suffered a heart attack and as such, Palmer remained by Ducky's bedside.
As a result, Ducky was sidelined to recover from his heart while Palmer became the acting Medical Examiner in charge. In Phoenix (episode), Ducky was finally cleared to resume his duties as the official Medical Examiner much to Palmer's own relief.
NCIS Season 11
NCIS Season 12
As the Assistant to the Chief Medical Examiner for NCIS, Jimmy is often tasked with helping Ducky perform numerous autopsies on the many bodies that NCIS often get to determine if the deaths were natural or as a result of murder.
Relationships with Colleagues
Unlike Gerald in season one, Jimmy is actually interested in Ducky's stories, often asking about what happened next. Jimmy also seems somewhat interested in Ducky's habit of chatting with the dead bodies. When Jimmy asked why, Ducky told him that it helped to build a relationship with the corpses. Jimmy later tried it himself, until he was scared by Abby.
Jimmy cares deeply for Doctor Mallard, which he demonstrated in his own way at the conclusion of the Season 6 episode, Broken Bird (episode). Jimmy related to Ducky why there is no J Street in Washington D.C., saying that the founding fathers did not want anyone to get lost. This scene also demonstrated that Jimmy had learned from Ducky's question at the beginning of the episode: "Why is there no J street in Washington?"
Jimmy also has a lot of respect for Ducky, and places a lot of importance on what Dr. Mallard thinks of him. When he and Dr. Mallard were re-assembling three dissected bodies, Jimmy went to a lot of trouble to explain why he placed a great toe with a corpse's hand (the person had lost his thumb, and subsequently had a toe-to-thumb transplant) because he didn't want Dr. Mallard thinking that he had mistaken a toe for a thumb.
Ducky's relationship with Jimmy is akin to the relationship that exists between a father and a son. In this way Ducky's and Jimmy's relationship parallels the relationship between Gibbs and DiNozzo, with the elder mentoring the younger: Ducky through his stories, and Gibbs through his rules. To a certain extent Jimmy is developing Ducky's trait of storytelling while working on cases, as demonstrated in the Season 6 episode, Dead Reckoning (episode) when Jimmy and Ducky engage in a discussion about harmony.
This was shown further in the Season 7 episode, Ignition (episode) when he talked about past experiences involving squirrels while performing an autopsy on a dead squirrel.
It was revealed in the Season 6 episode, Bounce (episode) that DiNozzo confides in Jimmy and asks of him a degree of guidance and advice, when he requires assistance with a case, very similar to how Ducky interacts with Gibbs in the Season 6 episode, Collateral Damage (episode).
DiNozzo's nickname for Jimmy is "Autopsy gremlin" which he calls him often and makes jokes around the name.
It is also implied that Jimmy and DiNozzo occasionally hang out outside of work, as Ducky asks Jimmy in the Season 3 episode, Silver War (episode), "Have you been hanging out with Agent DiNozzo again?" when Jimmy jokes about sleeping with women.
Jimmy had an intimate relationship with Michelle Lee, as demonstrated in the fourth season episodes In the Dark (episode) and Angel of Death (episode) In In the Dark (episode), Ducky found a bra inside a drawer. Jimmy then received a phone call which appeared to be from Michelle, asking if he had also found her panties.
However, Jimmy related to Gibbs and Director Vance during the mole investigation in the Season 6 episode, Last Man Standing (episode) that he ended the relationship because he felt he was being used which later turned out to be true.