Is it true that South Koreans are ultra racist?

In my opinion, yes, they are very xenophobic and racist, but you won't have much experience with it since you will only be here a month. It takes some time to realize whats going on in the country, and if you don't understand the language you won't pick up much of the negativity anyway. You will be looked down on somewhat because of your skin color, but you might not recognize it in such a short time. Also, Seoul is a little better maybe than some other places because it is somewhat metropolitan.