That's Mr. Jimmy Palmer

Jim (Jimmy) Palmer, portrayed by Brian Dietzen, and sometimes referred to by Tony as "Autopsy Gremlin", first appeared in the episode "Split Decision". After Gerald Jackson was incapacitated, Palmer became Mallard's medical assistant both in the field and in the morgue. In the episode "About Face", Jimmy became a central character of the episode who must recover his memory to find a suspect to the murder case and his attempted killer. He self-identifies as a sufferer of a "mild" case of diabetes mellitus in the episode "In The Dark". He is terrified of Gibbs. Part of the reason that Doctor Mallard and he are often not at the crime scene until well after Gibbs and his team arrive is related to Dr. Mallard's emphasis on Jimmy being a horrible driver and always getting lost, although Jimmy tries to defend himself by pointing out that Ducky is the one with the map. He was named after former Baltimore Orioles pitcher Jim Palmer, but does not like baseball. In many of the episodes, Jimmy is seen fraternizing with Michelle Lee. Often they make excuses for working late and are seen entering and exiting the underside of the autopsy table. In the episode "Last Man Standing", Palmer admits to Gibbs and Vance that he and Agent Lee had been "doing it" for a while. In the episode "The Good Wives Club" it is revealed that Jimmy is claustrophobic; when he is entering the enclosed hallway he is seen sweating profusely and when he has to go get the body bag he gets freaked out about having to go back through it. In the episode "About Face", as Jimmy was being hypnotized by Abby, it can be inferred that he has a foot/shoe fetish as he dreamily states in great detail about Ziva and Abby's footwear, instead of recalling information about the current case. The episode also leads us to believe that his mother's name is Eunice.

That's Gerald Jackson

Gerald Jackson, portrayed by Pancho Demmings, first appeared in the episode "Yankee White". Gerald Jackson was Dr. Mallard's original assistant until being incapacitated by Ari Haswari after being held hostage in the morgue. While recuperating from his shoulder gunshot wound, his position was filled by Jimmy Palmer. Approximately a year and a half later, a week before he was scheduled to return to work, Gerald was again captured by Ari Haswari in an effort to secure Dr. Mallard's attention. After Ari released him, Gerald had not been seen since and has been permanently replaced by Jimmy Palmer.