I also like Mary Story, My aunt Mary, Izy (beware of Engrish), but they're maybe a bit less rock-ish and more pop-ish? Hmm. Dear Cloud and Cloud Cuckoo Land -both two of my favorites- are more indie, imho.
I suggest you go to last.fm and run a search there. You'll see lots of recs based on similar bands/singers.
Galaxy Express are an excellent korean rock band. They play at club FF regularly and u can get their album from itunes. Its high tempo energetic music, one of the best live bands ive seen since ive been here.