That's Dr. Donald 'Ducky' Mallard

Dr. Donald "Ducky" Mallard, portrayed by David McCallum, is the Chief Medical Examiner at NCIS. Dr. Mallard is a Scottish-born doctor, who has been long-time friends with Gibbs, and underwent medical education at the University of Edinburgh Medical School, and served in the Royal Army Medical Corps. He has a "second talent", as Gibbs calls it, to be able to read people, which he expands in Season 4 by studying psychology. In cases without actual bodies, he assists by using his psychological training to decipher the clues left by the perpetrators. Dr. Mallard is an eccentric character who often talks to the deceased and rambles to the living with many long personal remembrances or historical accounts, but is a kind man at heart. He also calls co-workers by their full first names (ex: Abigail instead of Abby) - with the exceptions of Gibbs and medical assistant Jimmy Palmer, whom he addresses as Jethro and Mr. Palmer, respectively (although he does refer to Palmer by his first name, Jimmy, when concerned for him, as revealed in About Face). Although most of his time is spent in autopsy and going to crime scenes to examine bodies, he was sent on a highly important undercover mission in the episode "Blowback". He also spent some time in Afghanistan during the Soviet invasion, and in Bosnia during the Balkan conflict. Ducky and Gibbs have worked together for many years. When Gibbs was asked, "What did Ducky look like when he was younger?" he replied, "Illya Kuryakin" - the Russian spy played by McCallum in the 1960s television show The Man From U.N.C.L.E. Ducky drives a Morgan that he restored himself. In the episode "Hung Out to Dry," it is revealed that he has a nephew, though no further information follows. The ring tone on his cell phone features bagpipes playing "Scotland the Brave". He lived with his aging mother and their corgis until season 6. In the episode "Broken Bird", Ducky revealed his mother had moved out and had Alzheimer's disease.  Nina Foch, the actress who played Ducky's mother, died on December 5, 2008, necessitating the change. In the episode "Double Identity", it was revealed Ducky's mother had died. Her headstone indicated 1912-2010. Ducky is, so far, the only main character introduced in the JAG pilot episodes to miss an episode of NCIS when he was absent from "Legend (Part 2)".