I think I can clarify this one.Trunks680 wrote: 4. Perhaps she went back to Jason (or maybe Jason put out a hit on her). I felt that bringing back Moo Hyuk's girlfriend was a weak part of the final episodes as she did not have much relevance to the plot. By that time, Moo Hyuk's feelings had already moved past her and he was entirely devoted to Eun Chae. For the writers to suddenly bring her back only to discard of her character that quickly without a follow through is a sign of weak writing.
The reason the writer brought her back was to give Moo-Hyuk a chance, if even a very slim chance, to live. He had a choice - save himself, and stay with Eunchae, or die, and save Yoon's life. He chose to die. That was his way of making up for causing Yoon's accident in the first place, and for all of his attempts at getting revenge. This was his way of repenting, and finally becoming truly selfless - like Eunchae. He came full circle.
She was also brought back to clairfy Moo-Hyuk's feelings about her. I mean, he was basically dying for this girl. He truly and deeply loved her. But with her return, it showed that his feelings had changed, and that they had completely shifted towards Eunchae...as shown in the scene where she's singing to him on the couch, and he gets a flashback of Eunchae - the complete opposite of what had happened in the episode of his and Eunchae's first kiss. It cast away all doubts the audience might have had, and the doubts Moo-Hyuk might have had about his feelings.
Er...I had another point to make, but I forgot it. It was a really good one, too... Oh, well. I hope that helps.