Riding ktx is quite strange to me. I printed the ticket from my home country, but turned out, you just have to sit on your booked seat, you don’t have to show your ticket to anyone, I asked the koreans, and they told me, they just check the booked seat, if unreserved seat is sat by someone, they will ask and check that person.
So as long as you are sitting on the right seat, you are fine.
Be fast if you bring big suitcase, there is limited space for people to storage their suitcase.