I'm thinking about this too, I'm assuming the same theory as you do, he cheats with Ji Wook's ex-gf, and maybe they break up after that (?), that's why he's trying his hardest to be close to his friend again :)
I love Eun Hyuk's character, so I really hope he's not the person Ji Wook's girlfriend slept with. If that's the case, It would be really hard to forgive him. Not just for Ji Wook, but as a viewer, for me too.
@Panda I just don't see the Eun Hyuk character *as we know him* doing something like that when he so openly adores Ji Wook. The guys is saved in his phone as "My Ji Wookie" for heaven's sake. :P
I'm not saying it's definitely not him (it probably is), but I don't think it's a believable plot point at this juncture. Also, Ji Wook's hatred for him, while thorough, is not so dark as it would be for the person who slept with his wife/gf. We'll see, I guess.
I think it's totally believable. My theory is that the older guy is his father and after JCW's father died, he lived with them, adopted. Therefore, they would be brothers, in that sense. Eun Hyuk could have made the mistake of sleeping with JCW's gf in a moment of stupidity and regretted it, but JCW refuses to forgive him.