der Südkoreaner
South Korean man
A male person who comes from South Korea or has South Korean citizenship.
Der neue Mitarbeiter in unserer Abteilung ist Südkoreaner.
The new employee in our department is South Korean.

South Korean
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South Korean man
A male person who comes from South Korea or has South Korean citizenship.
Der neue Mitarbeiter in unserer Abteilung ist Südkoreaner.
The new employee in our department is South Korean.

The word is made up of three parts: 'Süd-' (south, compass direction), 'Korea' (country), and the ending '-aner'. The ending '-aner' is often used for male inhabitants of countries or continents.
Ein Amerikaner und ein Südkoreaner unterhielten sich über ihre Kulturen.
An American and a South Korean talked about their cultures.
The related adjective is 'südkoreanisch' (South Korean). It is used to describe things that come from South Korea or relate to the country.
Die südkoreanische Küche ist für ihre scharfen Gerichte bekannt.
South Korean cuisine is known for its spicy dishes.
Nationality terms like 'der Südkoreaner' (the South Korean man) are treated like normal nouns. They appear with an article and are declined in the sentence.
Ich habe einem Südkoreaner den Weg zum Bahnhof erklärt.
I explained the way to the train station to a South Korean man.
Korean
'der Koreaner' and 'der Südkoreaner' both refer to a male person connected to Korea. 'der Koreaner' is more general and can refer to Korea as a whole, to origin, ancestry, or culture. 'der Südkoreaner' is more specific and means a male person from South Korea or with South Korean citizenship. The words can be confused because South Koreans are, of course, also Koreans. But if the country or citizenship is important, 'Südkoreaner' is more precise.

Im Urlaub habe ich einen Südkoreaner kennengelernt, der aus Seoul kommt.
On vacation, I met a South Korean man who comes from Seoul.
Der Koreaner in meinem Sprachkurs erzählt oft von seiner Familie.
The Korean man in my language course often talks about his family.