Here's my interpretation of some of the situation and mind games so far:
basically, I think Si-Won came back to Korea because of him; it was starting to suck in America, and she realized she had something great (Jeonghan) going on back home. I'm not sure if she was thinking of having something romantic with him at first, or just wanted to chill like old times though. Then she gets back, stuff happens, and then comes the scene where Jeonghan says he didn't want Song-Hua to think he's a horny teenager. I think that the reason Si-won got into that bathtub/let-him-touch-her-on-the-bed situation (aside from drunkness), as well as why she was disappointed when he wanted to put it in, is that she wanted him to back off like he did with Song-Hua, because she had a similar thought as that one other dude that was sleeping with Song-Hua; something along the lines of "A guy that finds you precious would rather dress you up than take your clothes off."