The South Korean military cannot confirm whether the person in question is still alive. It has informed the North Koreans of the incident via a military hotline. In 2020, a South Korean government employee who had disappeared from a ship was shot dead by the North Korean military while floating on a raft in the water. His body was then burned by the North Korean authorities. North Korean leader Kim Jong-un later apologized.
It’s unusual for someone to defect from South to North Korea. From the north to the south is more common, but usually North Koreans first flee to China and then travel to South Korea via yet another country. The demilitarized zone is a dangerous area with many landmines, which is heavily guarded by soldiers on both sides.