I don't know what the term would be in Hangul, but the US soldiers stationed in Korea typically referred to them as "ROKs" (Republic of Korea). If denoting individual branches, it would be "ROK Army", "ROK Marines", "ROK Air Force", etc., just as we'd refer to them as "ROK soldier", "ROK officer", etc. In addressing the rank of ROK military personnel, we'd more often refer to them by the US equivalent to their rank, rather than the Hangul name for their rank.
There's also the instance of KATUSA soldiers - Korean Augmentees To the United States Army - ROK Army personnel assigned to US units to act as interpreters and such.
A Korean.
A Korean. Seoul is the capital and largest city of S. Korea. So, much like someone from New York City is an American, so too is a person from Seoul a Korean.
An African American soldier on the frontier is called a Buffalo Soldier.
A down under soldier is called an anzac.
A soldier in the U. S. Army who fights with a rifle is called a weaponeer. A U.S. Marine is called a rifleman.
A person who goes to war is called a soldier. They may be a volunteer or a draftee, but they are still a soldier.
Someone from Seoul Korea is generally called "Korean", much like someone from Los Angeles is called "American".
A mounted soldier is called a knight. It begins with the letter k.
German Soldier or an Waffen SS Soldier.
Absent Without Leave
Citizen-soldier
A Spanish soldier is called a Red Coat