South Korea:
1. All ethnic Koreans living in China are by law allowed to live in Korea. Most now live in Korea. Chinese nationals (Ethnic Koreans and Han Chinese) are by far the single biggest foreign group in Korea. High representation in the national crime rate and have many bad stereotypes. Left Wing/Right Wing Koreans universally dislike having them as neighbors and blame them for many national problems (Housing prices, using tax money for public services, crime).
2. Left/Right wing Koreans universally dislike living with them as neighbors, but politically it's much more divided. Left wingers (current ruling party) are politically pro-China and desire stronger political and economic ties with China, while trying to maintain US alliance (good luck with that lol). Right wing Koreans commonly believe China is secretly controlling and interfering with all government branches and accuse China of rigging elections electronically.
3. China is Korea's biggest economic partner, so it's not that simple to just tell China to fuck off and become USA's buddy.