For a domestic flight inside South Korea, carry an original identity document. Korean Air's current domestic-boarding guidance lists a passport among accepted photo IDs for adult passengers, including foreign travelers. If you are unsure whether another document will be accepted by your airline, use your passport as the safest default rather than relying on a photo or copy.
Do not assume that having a mobile boarding pass means you can arrive at the last minute. Korean Air currently says domestic check-in closes 30 minutes before departure, boarding begins about 20 minutes before departure, and boarding closes 10 minutes before departure. Other airlines can set different deadlines, and airport congestion can add time at bag drop and security.
Online check-in is useful, but it does not remove the airport deadline
Korean Air currently opens online check-in for domestic flights from 48 hours until 30 minutes before departure. Its guidance still tells passengers to reach the departure area at least 30 minutes before departure. Some travelers, including passengers whose special-service arrangements need confirmation and unaccompanied minors, may not be eligible for online check-in. Treat this as a Korean Air example and check the operating airline's own rule if you are flying another carrier.
If you are checking baggage, traveling with a child, pet, special equipment or another service request, allow more time because self-service options may be limited. Korea Airports Corporation also warns that self check-in and baggage-drop availability varies by airline and airport.
Best practice for foreign visitors: keep your passport in your hand baggage, confirm the operating airline and terminal, check that airline's domestic-flight cutoff on the day of travel, and arrive early enough to clear bag drop and security without relying on the final possible minute.