Any visitor who has an intention to into Korea shall have a valid visa wether it is for work, travel or study. Usually visa is attached into a passport. Anyway a valid visa does NOT gurantee that you may enter into Korea. During an entry inspection, the immigration officer might deny enterning into Korea.
Many times I visited immigration office and sign a documents that I will bear all the cost of etay. In case, the foreigner does not return back to other countries until the last day of permission, the guarantor shall be punished according tio Immigration Act.