Choe In-guk, 72, arrived in Pyongyang on Saturday and announced he would live there in a video published by North Korean media, following in the footsteps of his parents who made a switch of allegiance in 1986.
Such a move is highly unusual and is more frequently seen in the other direction. South Korea says more than 30,000 people have fled there since the 1950-53 Korean War to escape decades of repression and poverty in the North.
“He was always sorry he couldn't help his children financially,” Na said, noting that Choe had an estranged relationship with them and his wife.
Another acquaintance of Choe named Song Beom-du told the South's Dong-A Ilbo daily that the defector had said there was “nothing” he could do in South Korea and his life was lived “in pain”.
Choe's late father, Choe Dok Shin, was a retired general who fought in the Korean War and served as foreign minister during the presidency of Park Chung-hee, who led South Korea with an iron grip from 1961 to 1979.
In 1976 the senior Choe emigrated to the United States with his wife Ryu Mi Yong after his relationship with the Park government turned sour.
The couple then resettled in North Korea -- making them the highest-profile South Korean defectors to the reclusive state to date -- and lived among Pyongyang's elite.
Ryu reunited with her son in 2000 when she led the North Korean delegation to Seoul for a meeting of separated families there. She died in 2016.
Choe In-guk had made 12 authorised trips across the border since 2001, the South Korean government said, with the latest in November last year on the third anniversary of his mother's death.
South Korean citizens are not allowed to visit North Korea or contact North Koreans without the government's permission.
Seoul's unification ministry said Monday it was unaware of Choe's move until the video report was published, and was investigating how he had made the journey. (AFP)