As of 2006, the Ministry of Unification registered about 9,000 North Koreans in South Korea. Many thousands of them escaped from North to South as a result of the North Korean famine of 1990s.
North Koreans living in South Korea are often mistreated at schools, denied employment, and are subject to other kinds of ethnic issues due to their being from North Korea.
References
North Koreans in South Korea Wikipedia(Text) CC BY-SA