The ROK is our ally only on paper. Ask people on the street in South Korea who started the Korean War in 1950, and most will say America started it.
I'd noted some protests a few years ago over our military presence...
To tell you the truth, I have NO idea how the average South Korean actually regards the current US presence or their opinion about our involvement and sacrifice in preventing their enslavement during the Korean War.
IMO we ought to pull out our military by 75% across the globe. We've been un-compensated and un-appreciated for our trouble.